C++構造函數語義學——Default Constructor

時間 2020-04-22

標籤 c++ 構造函數語義學 default constructor 欄目 C&C++ 简体版

原文原文鏈接

Default Constructor詳解

一些C++的書籍中告訴咱們：若是不寫任何構造函數，那麼編譯器會爲咱們生成一個默認構造函數（default constructor）程序員

那麼，針對這個觀點，有兩個問題：ide

這句話是正確的嗎？
若是這句話是正確的，那麼編譯器爲咱們生成的default constructor裏有什麼動做嗎？

1 這句話是正確的嗎？

這句話大致是正確的，可是cppreference中有更詳細的說明：函數

隱式聲明的默認構造函數若不對類類型（struct、class 或 union）提供任何用戶聲明的構造函數，則編譯器將始終聲明一個做爲其類的 inline public 成員的默認構造函數。當存在用戶聲明的構造函數時，用戶仍能夠關鍵詞 default 強制編譯器自動生成本來隱式聲明的默認構造函數。this

上面這段話告訴咱們：只有在沒有任何用戶聲明的構造函數的狀況下，編譯器纔會聲明一個default constructor。換言之，只要有任何用戶聲明的構造函數（不必定要是default constructor），哪怕該構造函數不是default constructor，那麼編譯器就不會爲咱們生成default constructor（除了強制編譯器生成）lua

隱式定義的默認構造函數 若隱式聲明的默認構造函數未被定義爲棄置的，則當其被ODR 式使用時，它爲編譯器所定義（即生成函數體並編譯之），且它與擁有空函數體和空初始化器列表的用戶定義的構造函數有嚴格相同的效果。即它調用這個類的各基類和各非靜態成員的默認構造函數spa

這裏有一個條件，若是隱式聲明的默認構造函數未被定義爲棄置的，且當其被ODR式使用時，該隱式聲明的構造函數纔會生成函數體並被編譯3d

有沒有什麼證據能證實編譯器真的隱式聲明瞭一個默認構造函數呢？有，考慮以下的一個類：code

class A {};
複製代碼

這個類中沒有任何成員變量或者成員函數，此時打印出該類的相關信息以下：對象

// clang -cc1 -fdump-record-layouts -emit-llvm main.cpp 命令
*** Dumping AST Record Layout
         0 | class A (empty)
           | [sizeof=1, dsize=1, align=1,
           |  nvsize=1, nvalign=1]

*** Dumping IRgen Record Layout
Record: CXXRecordDecl 0x55c6626abc70 <main.cpp:1:1, line:3:1> line:1:7 referenced class A definition
|-DefinitionData pass_in_registers empty aggregate standard_layout trivially_copyable pod trivial literal has_constexpr_non_copy_move_ctor can_const_default_init
| |-**DefaultConstructor exists trivial constexpr defaulted_is_constexpr** 
| |-CopyConstructor simple trivial has_const_param implicit_has_const_param
| |-MoveConstructor exists simple trivial
| |-CopyAssignment trivial has_const_param needs_implicit implicit_has_const_param
| |-MoveAssignment exists simple trivial needs_implicit
| `-Destructor simple irrelevant trivial needs_implicit
|-CXXRecordDecl 0x55c6626abd88 <col:1, col:7> col:7 implicit class A
|-CXXConstructorDecl 0x55c6626ac020 <col:7> col:7 implicit used constexpr A 'void () noexcept' inline default trivial
| `-CompoundStmt 0x55c6626ac4b8 <col:7>
|-CXXConstructorDecl 0x55c6626ac158 <col:7> col:7 implicit constexpr A 'void (const A &)' inline default trivial noexcept-unevaluated 0x55c6626ac158
| `-ParmVarDecl 0x55c6626ac268 <col:7> col:7 'const A &'
`-CXXConstructorDecl 0x55c6626ac308 <col:7> col:7 implicit constexpr A 'void (A &&)' inline default trivial noexcept-unevaluated 0x55c6626ac308
  `-ParmVarDecl 0x55c6626ac418 <col:7> col:7 'A &&'

Layout: <CGRecordLayout
  LLVMType:%class.A = type { i8 }
  NonVirtualBaseLLVMType:%class.A = type { i8 }
  IsZeroInitializable:1
  BitFields:[
]>
複製代碼

DefaultConstructor exists trivial constexpr defaulted_is_constexpr這一行證實了這個類存在一個default constructor且是trivial的，而咱們並無聲明，那麼天然是編譯器爲咱們自動生成的繼承

2 隱式定義的默認構造函數裏發生了什麼？

在解決這個問題以前，首先有須要瞭解一些額外的知識：

default constructor是分爲trivial default constructor（平凡默認構造函數，即不進行任何構造動做）和non trivial default constructor（非平凡默認構造函數）

什麼狀況下default constructor是trivial的呢？cppreference中說明了這一點：

當下列各項所有爲真時，類 T 的默認構造函數爲平凡的（平凡默認構造函數是不進行任何動做的構造函數）：構造函數並不是用戶提供（即爲隱式定義或於其首個聲明中預置的） T 沒有虛成員函數 T 沒有虛基類 T 沒有擁有默認初始化器的非靜態數據成員。每一個 T 的直接基類都擁有平凡默認構造函數每一個類類型的非靜態成員都擁有平凡默認構造函數

注意上述條件中的第一點也說明了一個事實：若是構造函數是用戶定義的，那麼一定是非平凡的

再回到隱式定義的默認構造函數裏發生了什麼？這個問題，就須要從兩個角度來考慮了：

當編譯器隱式定義的默認構造函數是一個trivial default constructor時，那麼該trivial default constructor什麼也不作
那麼當編譯器隱式定義的默認構造函數是一個non-trivial default constructor時，該non-trivial default constructor調用這個類的各基類和各非靜態成員的默認構造函數

經過以上，咱們已經知道了編譯器在特定狀況下，真的會隱式定義一個default constructor，也知道了在哪些狀況下產生的default constructor是trivial的，那麼再來詳細的討論一下，什麼狀況下，編譯器生成的default constructor是non-trivial的？

帶有non-trivial default constructor的member class object

若是一個class A沒有任何的constructor，可是它內含一個member object，而該member object擁有non-trivial default constructor，那麼編譯器爲這個class隱式定義的default constructor是non-trivial。爲何呢？由於編譯器爲該class A產生的default constructor裏必需要調用member object的non-trivial default constructor，不然沒法完整的構造class A的對象

仍然用代碼來證實：

class B {
     private:
      int b;
     public:
      B() {}
    };
    
    class A {
     private:
        B b;
        int a;
    };
複製代碼

打印出class A的相關信息，以下：

*** Dumping AST Record Layout
          0 | class B
          0 |   int b
            | [sizeof=4, dsize=4, align=4,
            |  nvsize=4, nvalign=4]
 
 *** Dumping AST Record Layout
          0 | class A
          0 |   class B b
          0 |     int b
          4 |   int a
            | [sizeof=8, dsize=8, align=4,
            |  nvsize=8, nvalign=4]
 
 *** Dumping IRgen Record Layout
 Record: CXXRecordDecl 0x55ba37562c50 <main.cpp:1:1, line:6:1> line:1:7 referenced class B definition
 |-DefinitionData pass_in_registers standard_layout trivially_copyable has_user_declared_ctor can_const_default_init
 | |-**DefaultConstructor exists non_trivial user_provided**
 | |-CopyConstructor simple trivial has_const_param implicit_has_const_param
 | |-MoveConstructor exists simple trivial
 | |-CopyAssignment trivial has_const_param needs_implicit implicit_has_const_param
 | |-MoveAssignment exists simple trivial needs_implicit
 | `-Destructor simple irrelevant trivial
 |-CXXRecordDecl 0x55ba37562d68 <col:1, col:7> col:7 implicit referenced class B
 |-AccessSpecDecl 0x55ba37562df8 <line:2:2, col:9> col:2 private
 |-FieldDecl 0x55ba37562e38 <line:3:3, col:7> col:7 b 'int'
 |-AccessSpecDecl 0x55ba37562e88 <line:4:2, col:8> col:2 public
 |-CXXConstructorDecl 0x55ba37562f38 <line:5:3, col:8> col:3 used B 'void ()'
 | `-CompoundStmt 0x55ba37562ff8 <col:7, col:8>
 |-CXXDestructorDecl 0x55ba37563590 <line:1:7> col:7 implicit ~B 'void ()' inline default trivial noexcept-unevaluated 0x55ba37563590
 |-CXXConstructorDecl 0x55ba375636c8 <col:7> col:7 implicit constexpr B 'void (const B &)' inline default trivial noexcept-unevaluated 0x55ba375636c8
 | `-ParmVarDecl 0x55ba375637d8 <col:7> col:7 'const B &'
 `-CXXConstructorDecl 0x55ba37563878 <col:7> col:7 implicit constexpr B 'void (B &&)' inline default trivial noexcept-unevaluated 0x55ba37563878
   `-ParmVarDecl 0x55ba37563988 <col:7> col:7 'B &&'
 
 Layout: <CGRecordLayout
   LLVMType:%class.B = type { i32 }
   NonVirtualBaseLLVMType:%class.B = type { i32 }
   IsZeroInitializable:1
   BitFields:[
 ]>
 
 *** Dumping IRgen Record Layout
 Record: CXXRecordDecl 0x55ba37563008 <main.cpp:8:1, line:12:1> line:8:7 referenced class A definition
 |-DefinitionData pass_in_registers standard_layout trivially_copyable
 | |-**DefaultConstructor exists non_trivial**
 | |-CopyConstructor simple trivial has_const_param implicit_has_const_param
 | |-MoveConstructor exists simple trivial
 | |-CopyAssignment trivial has_const_param needs_implicit implicit_has_const_param
 | |-MoveAssignment exists simple trivial needs_implicit
 | `-Destructor simple irrelevant trivial needs_implicit
 |-CXXRecordDecl 0x55ba37563128 <col:1, col:7> col:7 implicit class A
 |-AccessSpecDecl 0x55ba375631b8 <line:9:2, col:9> col:2 private
 |-FieldDecl 0x55ba375631f0 <line:10:3, col:5> col:5 b 'B'
 |-FieldDecl 0x55ba37563258 <line:11:3, col:7> col:7 a 'int'
 |-CXXConstructorDecl 0x55ba375634b0 <line:8:7> col:7 implicit used A 'void () noexcept(false)' inline default
 | |-CXXCtorInitializer Field 0x55ba375631f0 'b' 'B'
 | | `-CXXConstructExpr 0x55ba37592090 <col:7> 'B' 'void ()'
 | `-CompoundStmt 0x55ba375920e8 <col:7>
 |-CXXConstructorDecl 0x55ba375639f8 <col:7> col:7 implicit constexpr A 'void (const A &)' inline default trivial noexcept-unevaluated 0x55ba375639f8
 | `-ParmVarDecl 0x55ba37591e30 <col:7> col:7 'const A &'
 `-CXXConstructorDecl 0x55ba37591ec8 <col:7> col:7 implicit constexpr A 'void (A &&)' inline default trivial noexcept-unevaluated 0x55ba37591ec8
   `-ParmVarDecl 0x55ba37591fd8 <col:7> col:7 'A &&'
 
 Layout: <CGRecordLayout
   LLVMType:%class.A = type { %class.B, i32 }
   NonVirtualBaseLLVMType:%class.A = type { %class.B, i32 }
   IsZeroInitializable:1
   BitFields:[
 ]>

 DefaultConstructor exists non_trivial user_provided
複製代碼

這一行中說明class B是用戶定義的一個default constructor，是non-trivial的

DefaultConstructor exists non_trivial
複製代碼

第二行中則說明class A的default constructor也是存在的且是non-trivial的。在這個non-trivial default constructor中，編譯器會調用class B的default constructor來初始化class A的對象中的b成分，可是不會初始化a（編譯器看來，初始化b是它的工做，可是初始化a是程序員的工做）

|-CXXConstructorDecl 0x55ba375634b0 <line:8:7> col:7 implicit used A 'void () noexcept(false)' inline default
 | |-CXXCtorInitializer Field 0x55ba375631f0 'b' 'B'
 | | `-CXXConstructExpr 0x55ba37592090 <col:7> 'B' 'void ()'
 | `-CompoundStmt 0x55ba375920e8 <col:7>
複製代碼

這一段則說明了在class A的default constructor中調用了class B的default constructor

若是，class A中提供了一個default constructor，那麼編譯器會怎麼辦呢？

class B {
      private:
       int b;
      public:
       B() {}
     };
     
     class A {
      private:
       B b;
       int a;
      public:
       A() {
         a = 0;
       }
     };

複製代碼

打印相關信息以下：

*** Dumping AST Record Layout
          0 | class B
          0 |   int b
            | [sizeof=4, dsize=4, align=4,
            |  nvsize=4, nvalign=4]
 
 *** Dumping AST Record Layout
          0 | class A
          0 |   class B b
          0 |     int b
          4 |   int a
            | [sizeof=8, dsize=8, align=4,
            |  nvsize=8, nvalign=4]
 
 *** Dumping IRgen Record Layout
 Record: CXXRecordDecl 0x5641e5965d20 <main.cpp:1:1, line:6:1> line:1:7 referenced class B definition
 |-DefinitionData pass_in_registers standard_layout trivially_copyable has_user_declared_ctor can_const_default_init
 | |-DefaultConstructor exists non_trivial user_provided
 | |-CopyConstructor simple trivial has_const_param implicit_has_const_param
 | |-MoveConstructor exists simple trivial
 | |-CopyAssignment trivial has_const_param needs_implicit implicit_has_const_param
 | |-MoveAssignment exists simple trivial needs_implicit
 | `-Destructor simple irrelevant trivial
 |-CXXRecordDecl 0x5641e5965e38 <col:1, col:7> col:7 implicit referenced class B
 |-AccessSpecDecl 0x5641e5965ec8 <line:2:2, col:9> col:2 private
 |-FieldDecl 0x5641e5965f08 <line:3:3, col:7> col:7 b 'int'
 |-AccessSpecDecl 0x5641e5965f58 <line:4:2, col:8> col:2 public
 |-CXXConstructorDecl 0x5641e5966008 <line:5:3, col:8> col:3 used B 'void ()'
 | `-CompoundStmt 0x5641e59660c8 <col:7, col:8>
 |-CXXConstructorDecl 0x5641e59664e8 <line:1:7> col:7 implicit constexpr B 'void (const B &)' inline default trivial noexcept-unevaluated 0x5641e59664e8
 | `-ParmVarDecl 0x5641e59665f8 <col:7> col:7 'const B &'
 |-CXXConstructorDecl 0x5641e5966698 <col:7> col:7 implicit constexpr B 'void (B &&)' inline default trivial noexcept-unevaluated 0x5641e5966698
 | `-ParmVarDecl 0x5641e59667a8 <col:7> col:7 'B &&'
 `-CXXDestructorDecl 0x5641e59953b8 <col:7> col:7 implicit ~B 'void ()' inline default trivial noexcept-unevaluated 0x5641e59953b8
 
 Layout: <CGRecordLayout
   LLVMType:%class.B = type { i32 }
   NonVirtualBaseLLVMType:%class.B = type { i32 }
   IsZeroInitializable:1
   BitFields:[
 ]>
 
 *** Dumping IRgen Record Layout
 Record: CXXRecordDecl 0x5641e59660d8 <main.cpp:8:1, line:16:1> line:8:7 referenced class A definition
 |-DefinitionData pass_in_registers standard_layout trivially_copyable has_user_declared_ctor can_const_default_init
 | |-DefaultConstructor exists non_trivial user_provided
 | |-CopyConstructor simple trivial has_const_param implicit_has_const_param
 | |-MoveConstructor exists simple trivial
 | |-CopyAssignment trivial has_const_param needs_implicit implicit_has_const_param
 | |-MoveAssignment exists simple trivial needs_implicit
 | `-Destructor simple irrelevant trivial needs_implicit
 |-CXXRecordDecl 0x5641e59661f8 <col:1, col:7> col:7 implicit referenced class A
 |-AccessSpecDecl 0x5641e5966288 <line:9:2, col:9> col:2 private
 |-FieldDecl 0x5641e59662c0 <line:10:3, col:5> col:5 b 'B'
 |-FieldDecl 0x5641e5966328 <line:11:3, col:7> col:7 referenced a 'int'
 |-AccessSpecDecl 0x5641e5966378 <line:12:2, col:8> col:2 public
 |-CXXConstructorDecl 0x5641e5966400 <line:13:3, line:15:3> line:13:3 used A 'void ()'
 | |-CXXCtorInitializer Field 0x5641e59662c0 'b' 'B'
 | | `-CXXConstructExpr 0x5641e5966818 <col:3> 'B' 'void ()'
 | `-CompoundStmt 0x5641e59668f0 <col:7, line:15:3>
 |   `-BinaryOperator 0x5641e59668d0 <line:14:5, col:9> 'int' lvalue '='
 |     |-MemberExpr 0x5641e5966880 <col:5> 'int' lvalue ->a 0x5641e5966328
 |     | `-CXXThisExpr 0x5641e5966870 <col:5> 'A *' implicit this
 |     `-IntegerLiteral 0x5641e59668b0 <col:9> 'int' 0
 |-CXXConstructorDecl 0x5641e5966b18 <line:8:7> col:7 implicit constexpr A 'void (const A &)' inline default trivial noexcept-unevaluated 0x5641e5966b18
 | `-ParmVarDecl 0x5641e5995348 <col:7> col:7 'const A &'
 `-CXXConstructorDecl 0x5641e59954c8 <col:7> col:7 implicit constexpr A 'void (A &&)' inline default trivial noexcept-unevaluated 0x5641e59954c8
   `-ParmVarDecl 0x5641e59955d8 <col:7> col:7 'A &&'
 
 Layout: <CGRecordLayout
   LLVMType:%class.A = type { %class.B, i32 }
   NonVirtualBaseLLVMType:%class.A = type { %class.B, i32 }
   IsZeroInitializable:1
   BitFields:[
 ]>

 |-CXXConstructorDecl 0x5641e5966400 <line:13:3, line:15:3> line:13:3 used A 'void ()'
 | |-CXXCtorInitializer Field 0x5641e59662c0 'b' 'B'
 | | `-CXXConstructExpr 0x5641e5966818 <col:3> 'B' 'void ()'
複製代碼

能夠發現，在咱們定義的default constructor中，也調用了class B的default constructor，且這個調用動做發生在 a = 0;這條語句以前，然而咱們並無寫呀，這即是編譯器自動安插的調用

也就是說，在這種狀況下，編譯器會擴充已經存在constructor，在其中安插一些代碼，使得在該constgructor中全部代碼執行以前，先調用必要的default constructor

若是class A提供了其餘的constructors，可是沒有提供default constructor，那麼編譯器還會爲咱們生成default consturctor嗎？

這個以前已經說過了，答案是不會。依然用事實證實：

class B {
      private:
       int b;
      public:
       B() {}
     };
     
     class A {
      private:
       B b;
       int a;
      public:
       A(int _a) : a(_a) {}
     };
     
     int main() {
       A a(1);
     }

複製代碼

*** Dumping AST Record Layout
          0 | class B
          0 |   int b
            | [sizeof=4, dsize=4, align=4,
            |  nvsize=4, nvalign=4]
 
 *** Dumping AST Record Layout
          0 | class A
          0 |   class B b
          0 |     int b
          4 |   int a
            | [sizeof=8, dsize=8, align=4,
            |  nvsize=8, nvalign=4]
 
 *** Dumping IRgen Record Layout
 Record: CXXRecordDecl 0x55df47367d20 <main.cpp:1:1, line:6:1> line:1:7 referenced class B definition
 |-DefinitionData pass_in_registers standard_layout trivially_copyable has_user_declared_ctor can_const_default_init
 | |-DefaultConstructor exists non_trivial user_provided
 | |-CopyConstructor simple trivial has_const_param implicit_has_const_param
 | |-MoveConstructor exists simple trivial
 | |-CopyAssignment trivial has_const_param needs_implicit implicit_has_const_param
 | |-MoveAssignment exists simple trivial needs_implicit
 | `-Destructor simple irrelevant trivial
 |-CXXRecordDecl 0x55df47367e38 <col:1, col:7> col:7 implicit referenced class B
 |-AccessSpecDecl 0x55df47367ec8 <line:2:2, col:9> col:2 private
 |-FieldDecl 0x55df47367f08 <line:3:3, col:7> col:7 b 'int'
 |-AccessSpecDecl 0x55df47367f58 <line:4:2, col:8> col:2 public
 |-CXXConstructorDecl 0x55df47368008 <line:5:3, col:8> col:3 used B 'void ()'
 | `-CompoundStmt 0x55df473680c8 <col:7, col:8>
 |-CXXConstructorDecl 0x55df47368628 <line:1:7> col:7 implicit constexpr B 'void (const B &)' inline default trivial noexcept-unevaluated 0x55df47368628
 | `-ParmVarDecl 0x55df47368738 <col:7> col:7 'const B &'
 |-CXXConstructorDecl 0x55df473687d8 <col:7> col:7 implicit constexpr B 'void (B &&)' inline default trivial noexcept-unevaluated 0x55df473687d8
 | `-ParmVarDecl 0x55df473688e8 <col:7> col:7 'B &&'
 `-CXXDestructorDecl 0x55df473978a8 <col:7> col:7 implicit ~B 'void ()' inline default trivial noexcept-unevaluated 0x55df473978a8
 
 Layout: <CGRecordLayout
   LLVMType:%class.B = type { i32 }
   NonVirtualBaseLLVMType:%class.B = type { i32 }
   IsZeroInitializable:1
   BitFields:[
 ]>
 
 *** Dumping IRgen Record Layout
 Record: CXXRecordDecl 0x55df473680d8 <main.cpp:8:1, line:14:1> line:8:7 referenced class A definition
 |-DefinitionData pass_in_registers standard_layout trivially_copyable has_user_declared_ctor can_const_default_init
 | |-DefaultConstructor
 | |-CopyConstructor simple trivial has_const_param implicit_has_const_param
 | |-MoveConstructor exists simple trivial
 | |-CopyAssignment trivial has_const_param needs_implicit implicit_has_const_param
 | |-MoveAssignment exists simple trivial needs_implicit
 | `-Destructor simple irrelevant trivial needs_implicit
 |-CXXRecordDecl 0x55df473681f8 <col:1, col:7> col:7 implicit referenced class A
 |-AccessSpecDecl 0x55df47368288 <line:9:2, col:9> col:2 private
 |-FieldDecl 0x55df473682c0 <line:10:3, col:5> col:5 b 'B'
 |-FieldDecl 0x55df47368328 <line:11:3, col:7> col:7 a 'int'
 |-AccessSpecDecl 0x55df47368378 <line:12:2, col:8> col:2 public
 |-CXXConstructorDecl 0x55df473684b8 <line:13:3, col:22> col:3 used A 'void (int)'
 | |-ParmVarDecl 0x55df473683b8 <col:5, col:9> col:9 used _a 'int'
 | |-CXXCtorInitializer Field 0x55df473682c0 'b' 'B'
 | | `-CXXConstructExpr 0x55df47368958 <col:3> 'B' 'void ()'
 | |-CXXCtorInitializer Field 0x55df47368328 'a' 'int'
 | | `-ImplicitCastExpr 0x55df473685c0 <col:17> 'int' <LValueToRValue>
 | |   `-DeclRefExpr 0x55df47368580 <col:17> 'int' lvalue ParmVar 0x55df473683b8 '_a' 'int'
 | `-CompoundStmt 0x55df473689b8 <col:21, col:22>
 |-CXXConstructorDecl 0x55df47397720 <line:8:7> col:7 implicit constexpr A 'void (const A &)' inline default trivial noexcept-unevaluated 0x55df47397720
 | `-ParmVarDecl 0x55df47397838 <col:7> col:7 'const A &'
 `-CXXConstructorDecl 0x55df473979b8 <col:7> col:7 implicit constexpr A 'void (A &&)' inline default trivial noexcept-unevaluated 0x55df473979b8
   `-ParmVarDecl 0x55df47397ac8 <col:7> col:7 'A &&'
 
 Layout: <CGRecordLayout
   LLVMType:%class.A = type { %class.B, i32 }
   NonVirtualBaseLLVMType:%class.A = type { %class.B, i32 }
   IsZeroInitializable:1
   BitFields:[
 ]>

 Record: CXXRecordDecl 0x55df473680d8 <main.cpp:8:1, line:14:1> line:8:7 referenced class A definition
 |-DefinitionData pass_in_registers standard_layout trivially_copyable has_user_declared_ctor can_const_default_init
 | |-DefaultConstructor
複製代碼

這幾行證實了，編譯器沒有爲class A生成default constructor

|-CXXConstructorDecl 0x55df473684b8 <line:13:3, col:22> col:3 used A 'void (int)'
 | |-ParmVarDecl 0x55df473683b8 <col:5, col:9> col:9 used _a 'int'
 | |-CXXCtorInitializer Field 0x55df473682c0 'b' 'B'
 | | `-CXXConstructExpr 0x55df47368958 <col:3> 'B' 'void ()'
複製代碼

這幾行證實了編譯器擴充了A的單參數constructor，在裏面安插了對B的default consturctor的調用

帶有non-trivial default constructor的Base class

若是一個沒有任何user_provided constructors的class派生自一個帶有default constructor的base class，那麼這個dervied class的default constructor會被編譯器自動合成出來且是non-trivial的

一個須要注意的地方是，上述成立的一個充分沒必要要條件是：base class的default constructor只有是non-tirvial時，dervied class被合成的這個default constructor纔是non-trivial的；具體緣由能夠回顧一下以前所說的平凡默認構造函數的所需條件

同理，若是derived class提供了defualt constructor或者其餘constructors的話，其結果也與第1點中所述狀況相似，讀者能夠自行實驗

帶有一個virtual function的class

若是一個沒有任何user_provided constructors的class中聲明（或繼承）了一個virtual function，那麼編譯器會自動爲該class生成一個default constructor，且該defalut constructor是non-trivial的

舉例證實：

// 定義了一個virtual function
    class A {
     public:
      virtual void vfunc() {}
    };
    
    // 繼承了一個virtual function
    class B: public A {
    };
複製代碼

打印相關信息：

*** Dumping AST Record Layout
          0 | class A
          0 |   (A vtable pointer)
            | [sizeof=8, dsize=8, align=8,
            |  nvsize=8, nvalign=8]
 
 *** Dumping AST Record Layout
          0 | class B
          0 |   class A (primary base)
          0 |     (A vtable pointer)
            | [sizeof=8, dsize=8, align=8,
            |  nvsize=8, nvalign=8]
 
 *** Dumping IRgen Record Layout
 Record: CXXRecordDecl 0x55d1c6dc4c50 <main.cpp:1:1, line:4:1> line:1:7 referenced class A definition
 |-DefinitionData polymorphic literal has_constexpr_non_copy_move_ctor can_const_default_init
 | |-DefaultConstructor exists non_trivial constexpr defaulted_is_constexpr
 | |-CopyConstructor simple non_trivial has_const_param implicit_has_const_param
 | |-MoveConstructor exists simple non_trivial
 | |-CopyAssignment non_trivial has_const_param implicit_has_const_param
 | |-MoveAssignment exists simple non_trivial
 | `-Destructor simple irrelevant trivial
 |-CXXRecordDecl 0x55d1c6dc4d68 <col:1, col:7> col:7 implicit class A
 |-AccessSpecDecl 0x55d1c6dc4df8 <line:2:2, col:8> col:2 public
 |-CXXMethodDecl 0x55d1c6dc4e98 <line:3:3, col:25> col:16 vfunc 'void ()' virtual
 | `-CompoundStmt 0x55d1c6dc53c0 <col:24, col:25>
 |-CXXMethodDecl 0x55d1c6dc4f98 <line:1:7> col:7 implicit constexpr operator= 'A &(const A &)' inline default noexcept-unevaluated 0x55d1c6dc4f98
 | `-ParmVarDecl 0x55d1c6dc50a8 <col:7> col:7 'const A &'
 |-CXXMethodDecl 0x55d1c6dc5148 <col:7> col:7 implicit constexpr operator= 'A &(A &&)' inline default noexcept-unevaluated 0x55d1c6dc5148
 | `-ParmVarDecl 0x55d1c6dc5258 <col:7> col:7 'A &&'
 |-CXXDestructorDecl 0x55d1c6dc52e0 <col:7> col:7 implicit ~A 'void ()' inline default trivial noexcept-unevaluated 0x55d1c6dc52e0
 |-CXXConstructorDecl 0x55d1c6df4028 <col:7> col:7 implicit used constexpr A 'void () noexcept' inline default
 | `-CompoundStmt 0x55d1c6df4738 <col:7>
 |-CXXConstructorDecl 0x55d1c6df4100 <col:7> col:7 implicit constexpr A 'void (const A &)' inline default noexcept-unevaluated 0x55d1c6df4100
 | `-ParmVarDecl 0x55d1c6df4218 <col:7> col:7 'const A &'
 `-CXXConstructorDecl 0x55d1c6df4288 <col:7> col:7 implicit constexpr A 'void (A &&)' inline default noexcept-unevaluated 0x55d1c6df4288
   `-ParmVarDecl 0x55d1c6df4398 <col:7> col:7 'A &&'
 
 Layout: <CGRecordLayout
   LLVMType:%class.A = type { i32 (...)** }
   NonVirtualBaseLLVMType:%class.A = type { i32 (...)** }
   IsZeroInitializable:1
   BitFields:[
 ]>
 
 *** Dumping IRgen Record Layout
 Record: CXXRecordDecl 0x55d1c6dc53d0 <main.cpp:7:1, line:8:1> line:7:7 referenced class B definition
 |-DefinitionData polymorphic literal has_constexpr_non_copy_move_ctor can_const_default_init
 | |-DefaultConstructor exists non_trivial constexpr defaulted_is_constexpr
 | |-CopyConstructor simple non_trivial has_const_param implicit_has_const_param
 | |-MoveConstructor exists simple non_trivial
 | |-CopyAssignment non_trivial has_const_param implicit_has_const_param
 | |-MoveAssignment exists simple non_trivial
 | `-Destructor simple irrelevant trivial
 |-public 'A'
 |-CXXRecordDecl 0x55d1c6dc5528 <col:1, col:7> col:7 implicit class B
 |-CXXMethodDecl 0x55d1c6dc5618 <col:7> col:7 implicit constexpr operator= 'B &(const B &)' inline default noexcept-unevaluated 0x55d1c6dc5618
 | `-ParmVarDecl 0x55d1c6dc5728 <col:7> col:7 'const B &'
 |-CXXMethodDecl 0x55d1c6dc57c8 <col:7> col:7 implicit constexpr operator= 'B &(B &&)' inline default noexcept-unevaluated 0x55d1c6dc57c8
 | `-ParmVarDecl 0x55d1c6dc58d8 <col:7> col:7 'B &&'
 |-CXXDestructorDecl 0x55d1c6dc5960 <col:7> col:7 implicit ~B 'void ()' inline default trivial noexcept-unevaluated 0x55d1c6dc5960
 |-CXXConstructorDecl 0x55d1c6df3f30 <col:7> col:7 implicit used constexpr B 'void () noexcept' inline default
 | |-CXXCtorInitializer 'A'
 | | `-CXXConstructExpr 0x55d1c6df4748 <col:7> 'A' 'void () noexcept'
 | `-CompoundStmt 0x55d1c6df47b0 <col:7>
 |-CXXConstructorDecl 0x55d1c6df4408 <col:7> col:7 implicit constexpr B 'void (const B &)' inline default noexcept-unevaluated 0x55d1c6df4408
 | `-ParmVarDecl 0x55d1c6df4518 <col:7> col:7 'const B &'
 `-CXXConstructorDecl 0x55d1c6df4588 <col:7> col:7 implicit constexpr B 'void (B &&)' inline default noexcept-unevaluated 0x55d1c6df4588
   `-ParmVarDecl 0x55d1c6df4698 <col:7> col:7 'B &&'
 
 Layout: <CGRecordLayout
   LLVMType:%class.B = type { %class.A }
   NonVirtualBaseLLVMType:%class.B = type { %class.A }
   IsZeroInitializable:1
   BitFields:[
 ]>

 Record: CXXRecordDecl 0x55d1c6dc4c50 <main.cpp:1:1, line:4:1> line:1:7 referenced class A definition
 |-DefinitionData polymorphic literal has_constexpr_non_copy_move_ctor can_const_default_init
 | |-DefaultConstructor exists non_trivial constexpr defaulted_is_constexpr
 
 Record: CXXRecordDecl 0x55d1c6dc53d0 <main.cpp:7:1, line:8:1> line:7:7 referenced class B definition
 |-DefinitionData polymorphic literal has_constexpr_non_copy_move_ctor can_const_default_init
 | |-DefaultConstructor exists non_trivial constexpr defaulted_is_constexpr
複製代碼

這幾行證實了以前的所述的觀點

與以前的狀況不一樣的時，這種狀況下所生成的default constructor，不只須要調用必須的基類的default constructor，還須要完成設定vptr的動做，才能知足虛機制的運行

與以前相同的是，若是提供了任何用戶聲明的constructors，那麼天然不會生成default constructor，編譯器會對現有的constrcutors進行擴充，來完成必要的工做

仍是舉個例子吧：

class A {
     public:
      virtual void vfunc() {}
      A(int a) : _a(a) {}
     private:
      int _a;
    };
    
    class B : public A {
     public:
      B(int a, int b) : A(a), _b(b) {}
     private:
      int _b;
    };
複製代碼

打印相關信息：

*** Dumping AST Record Layout
          0 | class A
          0 |   (A vtable pointer)
          8 |   int _a
            | [sizeof=16, dsize=12, align=8,
            |  nvsize=12, nvalign=8]
 
 *** Dumping AST Record Layout
          0 | class B
          0 |   class A (primary base)
          0 |     (A vtable pointer)
          8 |     int _a
         12 |   int _b
            | [sizeof=16, dsize=16, align=8,
            |  nvsize=16, nvalign=8]
 
 *** Dumping IRgen Record Layout
 Record: CXXRecordDecl 0x56506d274d60 <main.cpp:1:1, line:7:1> line:1:7 referenced class A definition
 |-DefinitionData polymorphic has_user_declared_ctor can_const_default_init
 | |-DefaultConstructor
 | |-CopyConstructor simple non_trivial has_const_param implicit_has_const_param
 | |-MoveConstructor exists simple non_trivial
 | |-CopyAssignment non_trivial has_const_param implicit_has_const_param
 | |-MoveAssignment exists simple non_trivial
 | `-Destructor simple irrelevant trivial
 |-CXXRecordDecl 0x56506d274e78 <col:1, col:7> col:7 implicit referenced class A
 |-AccessSpecDecl 0x56506d274f08 <line:2:2, col:8> col:2 public
 |-CXXMethodDecl 0x56506d274fa8 <line:3:3, col:25> col:16 vfunc 'void ()' virtual
 | `-CompoundStmt 0x56506d275700 <col:24, col:25>
 |-CXXConstructorDecl 0x56506d275138 <line:4:3, col:21> col:3 used A 'void (int)'
 | |-ParmVarDecl 0x56506d275060 <col:5, col:9> col:9 used a 'int'
 | |-CXXCtorInitializer Field 0x56506d275228 '_a' 'int'
 | | `-ImplicitCastExpr 0x56506d275750 <col:17> 'int' <LValueToRValue>
 | |   `-DeclRefExpr 0x56506d275710 <col:17> 'int' lvalue ParmVar 0x56506d275060 'a' 'int'
 | `-CompoundStmt 0x56506d275798 <col:20, col:21>
 |-AccessSpecDecl 0x56506d2751e8 <line:5:2, col:9> col:2 private
 |-FieldDecl 0x56506d275228 <line:6:3, col:7> col:7 _a 'int'
 |-CXXMethodDecl 0x56506d2752d8 <line:1:7> col:7 implicit operator= 'A &(const A &)' inline default noexcept-unevaluated 0x56506d2752d8
 | `-ParmVarDecl 0x56506d2753e8 <col:7> col:7 'const A &'
 |-CXXMethodDecl 0x56506d275488 <col:7> col:7 implicit operator= 'A &(A &&)' inline default noexcept-unevaluated 0x56506d275488
 | `-ParmVarDecl 0x56506d275598 <col:7> col:7 'A &&'
 |-CXXDestructorDecl 0x56506d275620 <col:7> col:7 implicit ~A 'void ()' inline default trivial noexcept-unevaluated 0x56506d275620
 |-CXXConstructorDecl 0x56506d2a4af0 <col:7> col:7 implicit constexpr A 'void (const A &)' inline default noexcept-unevaluated 0x56506d2a4af0
 | `-ParmVarDecl 0x56506d2a4c08 <col:7> col:7 'const A &'
 `-CXXConstructorDecl 0x56506d2a4c78 <col:7> col:7 implicit constexpr A 'void (A &&)' inline default noexcept-unevaluated 0x56506d2a4c78
   `-ParmVarDecl 0x56506d2a4d88 <col:7> col:7 'A &&'
 
 Layout: <CGRecordLayout
   LLVMType:%class.A = type <{ i32 (...)**, i32, [4 x i8] }>
   NonVirtualBaseLLVMType:%class.A.base = type <{ i32 (...)**, i32 }>
   IsZeroInitializable:1
   BitFields:[
 ]>
 
 *** Dumping IRgen Record Layout
 Record: CXXRecordDecl 0x56506d2757a8 <main.cpp:9:1, line:14:1> line:9:7 referenced class B definition
 |-DefinitionData polymorphic has_user_declared_ctor can_const_default_init
 | |-DefaultConstructor
 | |-CopyConstructor simple non_trivial has_const_param implicit_has_const_param
 | |-MoveConstructor exists simple non_trivial
 | |-CopyAssignment non_trivial has_const_param implicit_has_const_param
 | |-MoveAssignment exists simple non_trivial
 | `-Destructor simple irrelevant trivial
 |-public 'A'
 |-CXXRecordDecl 0x56506d275908 <col:1, col:7> col:7 implicit referenced class B
 |-AccessSpecDecl 0x56506d275998 <line:10:2, col:8> col:2 public
 |-CXXConstructorDecl 0x56506d275b68 <line:11:3, col:34> col:3 used B 'void (int, int)'
 | |-ParmVarDecl 0x56506d2759d8 <col:5, col:9> col:9 used a 'int'
 | |-ParmVarDecl 0x56506d275a58 <col:12, col:16> col:16 used b 'int'
 | |-CXXCtorInitializer 'A'
 | | `-CXXConstructExpr 0x56506d2a4e10 <col:21, col:24> 'A' 'void (int)'
 | |   `-ImplicitCastExpr 0x56506d2a4df8 <col:23> 'int' <LValueToRValue>
 | |     `-DeclRefExpr 0x56506d2a4aa0 <col:23> 'int' lvalue ParmVar 0x56506d2759d8 'a' 'int'
 | |-CXXCtorInitializer Field 0x56506d2a45d0 '_b' 'int'
 | | `-ImplicitCastExpr 0x56506d2a4ea8 <col:30> 'int' <LValueToRValue>
 | |   `-DeclRefExpr 0x56506d2a4e68 <col:30> 'int' lvalue ParmVar 0x56506d275a58 'b' 'int'
 | `-CompoundStmt 0x56506d2a4ef8 <col:33, col:34>
 |-AccessSpecDecl 0x56506d275c20 <line:12:2, col:9> col:2 private
 |-FieldDecl 0x56506d2a45d0 <line:13:3, col:7> col:7 _b 'int'
 |-CXXMethodDecl 0x56506d2a4678 <line:9:7> col:7 implicit operator= 'B &(const B &)' inline default noexcept-unevaluated 0x56506d2a4678
 | `-ParmVarDecl 0x56506d2a4788 <col:7> col:7 'const B &'
 |-CXXMethodDecl 0x56506d2a4828 <col:7> col:7 implicit operator= 'B &(B &&)' inline default noexcept-unevaluated 0x56506d2a4828
 | `-ParmVarDecl 0x56506d2a4938 <col:7> col:7 'B &&'
 |-CXXDestructorDecl 0x56506d2a49c0 <col:7> col:7 implicit ~B 'void ()' inline default trivial noexcept-unevaluated 0x56506d2a49c0
 |-CXXConstructorDecl 0x56506d2a5150 <col:7> col:7 implicit constexpr B 'void (const B &)' inline default noexcept-unevaluated 0x56506d2a5150
 | `-ParmVarDecl 0x56506d2a5268 <col:7> col:7 'const B &'
 `-CXXConstructorDecl 0x56506d2a52d8 <col:7> col:7 implicit constexpr B 'void (B &&)' inline default noexcept-unevaluated 0x56506d2a52d8
   `-ParmVarDecl 0x56506d2a53e8 <col:7> col:7 'B &&'
 
 Layout: <CGRecordLayout
   LLVMType:%class.B = type { %class.A.base, i32 }
   NonVirtualBaseLLVMType:%class.B = type { %class.A.base, i32 }
   IsZeroInitializable:1
   BitFields:[
 ]>
複製代碼

讀者想必應該可以本身尋找答案了

帶有一個virtual base class的class

爲了實現virtual base class的語義和功能，在derived class中必然須要進行一些額外的工做，這些工做是編譯器來完成的

若是存在任何user_provided constructors，那麼這些工做會被安插進入這些user_provided constructors中，若是沒有user_provided constructors，那麼編譯器會合成non-trivial default constructor來完成必要的工做（好比調用基類的default constructor、設定虛機制所必須的相關信息之類）

最後再舉個例子吧：

class A {
     private:
      int _a;
    };
    
    class B: public virtual A {
     private:
      int _b;
    };
複製代碼

打印相關信息：

*** Dumping AST Record Layout
          0 | class A
          0 |   int _a
            | [sizeof=4, dsize=4, align=4,
            |  nvsize=4, nvalign=4]
 
 *** Dumping AST Record Layout
          0 | class B
          0 |   (B vtable pointer)
          8 |   int _b
         12 |   class A (virtual base)
         12 |     int _a
            | [sizeof=16, dsize=16, align=8,
            |  nvsize=12, nvalign=8]
 
 *** Dumping IRgen Record Layout
 Record: CXXRecordDecl 0x557ad6558c50 <main.cpp:1:1, line:4:1> line:1:7 referenced class A definition
 |-DefinitionData pass_in_registers standard_layout trivially_copyable trivial literal
 | |-DefaultConstructor exists trivial
 | |-CopyConstructor simple trivial has_const_param implicit_has_const_param
 | |-MoveConstructor exists simple trivial
 | |-CopyAssignment trivial has_const_param implicit_has_const_param
 | |-MoveAssignment exists simple trivial
 | `-Destructor simple irrelevant trivial
 |-CXXRecordDecl 0x557ad6558d68 <col:1, col:7> col:7 implicit class A
 |-AccessSpecDecl 0x557ad6558df8 <line:2:2, col:9> col:2 private
 |-FieldDecl 0x557ad6558e38 <line:3:3, col:7> col:7 _a 'int'
 |-CXXMethodDecl 0x557ad6559368 <line:1:7> col:7 implicit constexpr operator= 'A &(const A &)' inline default trivial noexcept-unevaluated 0x557ad6559368
 | `-ParmVarDecl 0x557ad6559478 <col:7> col:7 'const A &'
 |-CXXMethodDecl 0x557ad6559518 <col:7> col:7 implicit constexpr operator= 'A &(A &&)' inline default trivial noexcept-unevaluated 0x557ad6559518
 | `-ParmVarDecl 0x557ad6559628 <col:7> col:7 'A &&'
 |-CXXDestructorDecl 0x557ad6559970 <col:7> col:7 implicit ~A 'void ()' inline default trivial noexcept-unevaluated 0x557ad6559970
 |-CXXConstructorDecl 0x557ad6586a08 <col:7> col:7 implicit used A 'void () noexcept' inline default trivial
 | `-CompoundStmt 0x557ad6587118 <col:7>
 |-CXXConstructorDecl 0x557ad6586c68 <col:7> col:7 implicit constexpr A 'void (const A &)' inline default trivial noexcept-unevaluated 0x557ad6586c68
 | `-ParmVarDecl 0x557ad6586d78 <col:7> col:7 'const A &'
 `-CXXConstructorDecl 0x557ad6586de8 <col:7> col:7 implicit constexpr A 'void (A &&)' inline default trivial noexcept-unevaluated 0x557ad6586de8
   `-ParmVarDecl 0x557ad6586ef8 <col:7> col:7 'A &&'
 
 Layout: <CGRecordLayout
   LLVMType:%class.A = type { i32 }
   NonVirtualBaseLLVMType:%class.A = type { i32 }
   IsZeroInitializable:1
   BitFields:[
 ]>
 
 *** Dumping IRgen Record Layout
 Record: CXXRecordDecl 0x557ad6558ea0 <main.cpp:6:1, line:9:1> line:6:7 referenced class B definition
 |-DefinitionData
 | |-DefaultConstructor exists non_trivial
 | |-CopyConstructor simple non_trivial has_const_param implicit_has_const_param
 | |-MoveConstructor exists simple non_trivial
 | |-CopyAssignment non_trivial has_const_param implicit_has_const_param
 | |-MoveAssignment exists simple non_trivial
 | `-Destructor simple irrelevant trivial
 |-virtual public 'A'
 |-CXXRecordDecl 0x557ad6559010 <col:1, col:7> col:7 implicit class B
 |-AccessSpecDecl 0x557ad65590a0 <line:7:2, col:9> col:2 private
 |-FieldDecl 0x557ad65590e0 <line:8:3, col:7> col:7 _b 'int'
 |-CXXMethodDecl 0x557ad6559188 <line:6:7> col:7 implicit operator= 'B &(const B &)' inline default noexcept-unevaluated 0x557ad6559188
 | `-ParmVarDecl 0x557ad6559298 <col:7> col:7 'const B &'
 |-CXXMethodDecl 0x557ad65596c8 <col:7> col:7 implicit operator= 'B &(B &&)' inline default noexcept-unevaluated 0x557ad65596c8
 | `-ParmVarDecl 0x557ad65597d8 <col:7> col:7 'B &&'
 |-CXXDestructorDecl 0x557ad6559860 <col:7> col:7 implicit ~B 'void ()' inline default trivial noexcept-unevaluated 0x557ad6559860
 |-CXXConstructorDecl 0x557ad6586910 <col:7> col:7 implicit used B 'void () noexcept' inline default
 | |-CXXCtorInitializer 'A'
 | | `-CXXConstructExpr 0x557ad6587128 <col:7> 'A' 'void () noexcept'
 | `-CompoundStmt 0x557ad6587190 <col:7>
 |-CXXConstructorDecl 0x557ad6586ae0 <col:7> col:7 implicit B 'void (const B &)' inline default noexcept-unevaluated 0x557ad6586ae0
 | `-ParmVarDecl 0x557ad6586bf8 <col:7> col:7 'const B &'
 `-CXXConstructorDecl 0x557ad6586f68 <col:7> col:7 implicit B 'void (B &&)' inline default noexcept-unevaluated 0x557ad6586f68
   `-ParmVarDecl 0x557ad6587078 <col:7> col:7 'B &&'
 
 Layout: <CGRecordLayout
   LLVMType:%class.B = type <{ i32 (...)**, i32, %class.A }>
   NonVirtualBaseLLVMType:%class.B.base = type <{ i32 (...)**, i32 }>
   IsZeroInitializable:1
   BitFields:[
 ]>

 Record: CXXRecordDecl 0x557ad6558ea0 <main.cpp:6:1, line:9:1> line:6:7 referenced class B definition
 |-DefinitionData
 | |-DefaultConstructor exists non_trivial
複製代碼

能夠看到class B存在default constructor且是non-trivial的

總結

若是沒有任何user_provided constructor，那麼編譯器會始終生成一個default constructor（反之，若是存在任何user_provided constrcutor，那麼除非強制生成，不然編譯器是不會生成defualt construcotor的）
編譯器生成的default constructor分trivial和non-trivial兩種

C++新手通常有兩個誤區：