c++ - 移动构造函数的意外调用

Question

我正在用上面的代码练习 C++ 移动语义代码，但是 MemoryBlock 的值 25 的移动构造函数被调用了两次。之后，连续两次调用MemoryBlock的析构函数。

代码如下:

#include <iostream>
#include <vector>

class MemoryBlock {
private:
    int length;
    int* data;

public:
    // Default constructor
    MemoryBlock() = default;

    // Constructor
    explicit MemoryBlock(int p_length) : length(p_length), data(new int[length]) {
        std::cout << "In MemoryBlock(int), length is " << length << ".\n";
    }

    // Destructor
    ~MemoryBlock() {
        std::cout << "~MemoryBlock() is called. Deleting resources.\n";

        delete[] data;
    }

    // Copy constructor
    MemoryBlock(const MemoryBlock& other) : length(other.length), data(new int[other.length]) {
        std::cout << "In MemoryBlock(const MemoryBlock&), length is " << length << ".\n";

        for (int i = 0; i < length; ++i) {
            data[i] = other.data[i];
        }
    }

    // Move constructor
    MemoryBlock(MemoryBlock&& other) noexcept : length(0), data(nullptr) {
        std::cout << "In MemoryBlock(MemoryBlock&&), length is " << other.length << ".\n";

        length = other.length;
        data = other.data;

        other.length = 0;
        other.data = nullptr; 
    }
};

int main() {
    std::vector<MemoryBlock> vec;

    vec.push_back(MemoryBlock(25)); // (1)
    vec.push_back(MemoryBlock(60)); // (2)

    std::cout << "----------------- End of main() -----------------\n";

    return 0;
}

(try it live)

结果:

In MemoryBlock(int), length is 25.            // #1: Generate temporary object
In MemoryBlock(MemoryBlock&&), length is 25.  // Move temporary object #1 to vec's node
~MemoryBlock() is called. Deleting resources. // Destruct temporary object #1
In MemoryBlock(int), length is 60.            // #2: Generate temporary object
In MemoryBlock(MemoryBlock&&), length is 60.  // Move temporary object #2 to vec's node
In MemoryBlock(MemoryBlock&&), length is 25.  // ??
~MemoryBlock() is called. Deleting resources. // ??
~MemoryBlock() is called. Deleting resources. // ??
----------------- End of main() -----------------
~MemoryBlock() is called. Deleting resources. // Destruct vec's node 60
~MemoryBlock() is called. Deleting resources. // Destruct vec's node 25

我想知道为什么调用标记为??的行。

Answer 1

您代码中的 Vector 最初容量为零。它的 Allocator 正在为位于 push_back 的一个对象分配内存。

在 vec.push_back(MemoryBlock(60)) 之后；//(2) 为 2 个元素分配了新的内存块，并将第一个元素移动到新的内存块。

使用 vec.reserve(2); 来避免这种情况。