了解C ++中的向量实例占用的空间 [英] Understanding the space occupied by an vector instance in C++

问题描述

我试图理解std :: vector实例占用的字节数。以下代码：

  vector< int> uno; 
 uno.push_back（1）; 
 uno.push_back（1）; 
 cout<<1：< sizeof（uno）<<bytes<< endl; 
 cout<< endl; 
 
 vector< bool> unos; 
 unos.push_back（true）; 
 cout<<2：< sizeof（unos）<<bytes<< endl; 
 cout<< endl; 
  

给我这个输出：

1 ：12个字节

2：20个字节

有人可以解释为什么向量< bool> 具有比向量< int> ？更多的字节。测量向量实例的大小（以字节为单位）的正确方法是...向量中添加每个成员的大小？

解决方案

在C ++中， sizeof 运算符在编译时被总是评估，因此每个向量< T& 将具有相同的大小（除非向量< T> 是如向量< bool> ）。

如果你想知道12个字节，那么可能是3个指针的大小，或者指针的大小和元素数和容量。实际数据从不存储在向量内。

如果要知道总使用了多少内存，好的近似是：

  sizeof（std :: vector< T>）+ my_vector.capacity 
  

这只是一个近似值，因为它不考虑堆上的记帐数据。 / p>

I'm trying to understand the amount of bytes occupied by an instance of std::vector. The following code:

    vector <int>uno;
    uno.push_back(1);
    uno.push_back(1);
    cout <<"1: "<< sizeof(uno)<<" bytes"<<endl;
    cout << endl;

    vector <bool>unos;
    unos.push_back(true);
    cout <<"2: "<< sizeof(unos)<<" bytes"<<endl;
    cout << endl;

gives me this output:

1: 12 bytes

2: 20 bytes

Can someone explain me why the size of the vector<bool> has more bytes than the vector<int>?. And what is the correct way to measure the size in bytes of an vector instance... by adding the size of every member in the vector?

解决方案

In C++, the sizeof operator is always evaluated at compile time, so every vector<T> will have the same size (unless vector<T> is a specialization like vector<bool>).

In case you are wondering about the 12 bytes, that is probably the size of 3 pointers, or alternatively, the size of a pointer and an element count and a capacity. The real data is never stored inside the vector.

If you want to know how much memory is used total, a good approximation is:

sizeof(std::vector<T>) + my_vector.capacity() * sizeof(T)

This is only an approximation because it does not take into account book-keeping data on the heap.

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