c++官方文檔-按值傳遞和按引用傳遞

#include<stdio.h>
#include<iostream>
#include<queue>
#include<map>
#include<memory.h>
#include <math.h>
#include <stdlib.h>
#include <algorithm>
#include <climits>
#include <sstream>
#include <cstdlib>
using namespace std;

/**
 * Calling a function with parameters taken by value causes copies of the values to be made.
 * This is a relatively inexpensive operation for fundamental types such as int,
 * but if the parameter is of a large compound type,
 * it may result on certain overhead.
 * For example, consider the following function:
 */
/**
 *調用傳值函數的時候複製值到函數棧上,這個操做對於基本類型(like int)來講花費不多.
 *可是若是參數大的複合類型,花費就很大,好比下面這個函數.
 *函數的參數是倆個string(按值傳遞),返回的結果是倆個字符串拼接.
 *按值傳遞中,複製a和b的內容給函數,若是參數是長的字符串,這意味着調用函數時複製了大量的數據
 */
string concatenate(string a, string b)
{
    return a + b;
}
/**
 * 比較好的方式以下
 * 按引用傳遞不須要複製數據.函數操做直接在原始字符上(只是給它起了一個別名).
 *頂多這意味着給函數傳遞某些指針.
 *
 */
string concatenate2(string& a, string& b)
{
    return a + b;
}

/**
 * 按引用傳遞可能被調用的函數會修改引用的內容,咱們能夠這樣解決
 */
string concatenate3(const string& a,const string& b)
{
    return a+b;
}
/**
 * that for most fundamental types,
 * there is no noticeable difference in efficiency, and in some cases,
 * const references may even be less efficient!
 *
 * 對於基本類型,效率並不會有多大差異,在某些狀況下,常量引用可能效率更低
 */

/**
 * 參數的默認值
 */
int divide(int a,int b=2)
{
    int r;
    r = a/b;
    return (r);
}
int main(const int argc, char** argv)
{
    //from cstdlib
    cout<<divide(12)<<endl;
    cout<<divide(20,4)<<endl;
    return EXIT_SUCCESS;
}