C++引用到底占用空间吗？

Aheaboy

已于 2023-05-11 14:50:52 修改

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文章标签： c++ 开发语言算法

于 2023-04-27 10:52:47 首次发布

本文链接：https://blog.youkuaiyun.com/Aheaboy/article/details/130401835

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这是个伪命题。
占也不占，不占也占，占还是不占，需要加个前提。
是物理上的内存，还是逻辑上的。在绝对物理上，一定是占的，不可能为0，在逻辑上，可能不占。

内部原理

编译时

C++ 引用变量的内部机制实际上是通过指针来实现的。

int a = 10;
int& b = a;
//在编译时，编译器会将第二行代码转换为以下代码：
int* b = &a;

使用时

引用在使用时会自动解引用。
引用变量在内部实现上是一个指针变量，在使用时可以像普通变量一样进行访问，无需使用指针运算符进行解引用操作

引用占用内存的例子

#include <iostream>
using namespace std;

void test1(int a, int &b, int c, int d, int e, int f, int g, int h, int i, int j, int k, int l, int m, int n, int &o, int p)
{
    cout << &a << "\n";
    cout << &b << "\n";
    cout << &c << "\n";
    cout << &d << "\n";
    //--
    cout << &e << "\n";
    cout << &f << "\n";
    cout << &g << "\n";
    cout << &h << "\n";
    //--
    cout << &i << "\n";
    cout << &j << "\n";
    cout << &k << "\n";
    cout << &l << "\n";
    //--
    cout << &m << "\n";
    cout << &n << "\n";
    cout << &o << "\n";
    cout << &p << "\n";
}
void test2(int a, int b, int c, int d, int e, int f, int g, int h, int i, int j, int k, int l, int m, int n, int o, int p)
{
    cout << &a << "\n";
    cout << &b << "\n";
    cout << &c << "\n";
    cout << &d << "\n";
    //--
    cout << &e << "\n";
    cout << &f << "\n";
    cout << &g << "\n";
    cout << &h << "\n";
    //--
    cout << &i << "\n";
    cout << &j << "\n";
    cout << &k << "\n";
    cout << &l << "\n";
    //--
    cout << &m << "\n";
    cout << &n << "\n";
    cout << &o << "\n";
    cout << &p << "\n";
}
int main()
{
    int x = 5;
    int y = 6;
    test1(1, x, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, y, 16);

    cout << "---"<< "\n";
    test2(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16);
    return 0;
}
//输出
0x332a5ff620
0x332a5ff6ac
0x332a5ff630
0x332a5ff638
0x332a5ff640
0x332a5ff648
0x332a5ff650
0x332a5ff658
0x332a5ff660
0x332a5ff668
0x332a5ff670
0x332a5ff678
0x332a5ff680
0x332a5ff688
0x332a5ff6a8
0x332a5ff698
---
0x332a5ff620
0x332a5ff628
0x332a5ff630
0x332a5ff638
0x332a5ff640
0x332a5ff648
0x332a5ff650
0x332a5ff658
0x332a5ff660
0x332a5ff668
0x332a5ff670
0x332a5ff678
0x332a5ff680
0x332a5ff688
0x332a5ff690
0x332a5ff698

在这里插入图片描述
我的系统是64位的，指针变量占用8个字节的空间，查询代码如下：

#include <iostream>
using namespace std;
/**
 * 基本类型占用的内存空间
*/
int main() {
    std::cout << "Size of int: " << sizeof(int) << " bytes" << std::endl;
    std::cout << "Size of short: " << sizeof(short) << " bytes" << std::endl;
    std::cout << "Size of long: " << sizeof(long) << " bytes" << std::endl;
    std::cout << "Size of long long: " << sizeof(long long) << " bytes" << std::endl;
    std::cout << "Size of float: " << sizeof(float) << " bytes" << std::endl;
    std::cout << "Size of double: " << sizeof(double) << " bytes" << std::endl;
    std::cout << "Size of long double: " << sizeof(long double) << " bytes" << std::endl;
    std::cout << "Size of char: " << sizeof(char) << " bytes" << std::endl;
    std::cout << "Size of bool: " << sizeof(bool) << " bytes" << std::endl;
    cout<<"-------"<<"\n";
    std::cout << "Size of int*: " << sizeof(int*) << " bytes" << std::endl;
    std::cout << "Size of short*: " << sizeof(short*) << " bytes" << std::endl;
    std::cout << "Size of long*: " << sizeof(long*) << " bytes" << std::endl;
    std::cout << "Size of long long*: " << sizeof(long long*) << " bytes" << std::endl;
    std::cout << "Size of float*: " << sizeof(float*) << " bytes" << std::endl;
    std::cout << "Size of double*: " << sizeof(double*) << " bytes" << std::endl;
    std::cout << "Size of long double*: " << sizeof(long double*) << " bytes" << std::endl;
    std::cout << "Size of char*: " << sizeof(char*) << " bytes" << std::endl;
    std::cout << "Size of bool*: " << sizeof(bool*) << " bytes" << std::endl;
    return 0;
}

在这里插入图片描述

都是sizeof 惹的祸

之所以会有这个占不占内存的疑问。多数是因为sizeof的时候无法读取到引用占用的空间。
这就给人一种错觉，就是引用不占用内存空间。

为什么引用无法sizeof

sizeof无法读取引用所占用的空间，即使是引用内部表示为指针的内存占用。重点在于无法读取，而非不占用。

无法读取的原因就在于：
引用的自动解引用

每当我们sizeof(引用)的时候，实际上是sizeof(*指针)，而sizeof指针，则是sizeof(指针)。
所以sizeof(引用)得到的都是代表该引用的指针，指针解引用后的值。

注意：认真理解以下代码的两行带有<等同于>的注释


#include <iostream>
using namespace std;

int main() {
  #include <iostream>
using namespace std;

int main() {
    int a = 10;
    int& b = a; // 等同于 int* const d = &a; b = *d;
    int* c = &a;

    cout << "sizeof(b) = " << sizeof(b) << endl; // 输出 sizeof(b) = 4
    //sizeof(b) 等同于 sizeof(*d) 但是实现sizeof(d) 
    cout << "sizeof(c) = " << sizeof(c) << endl; // 输出 sizeof(c) = 8

    return 0;
}

}