不同场景下的构造函数调用

本文为对不同场景下的构造函数调用进行跟踪。

构造函数

默认情况下，在 C++ 之后至少存在六个函数 默认构造/析构函数，复制构造/复制赋值，移动构造/移动赋值。以下代码观测发生调用的场景

#include <iostream>

struct Foo {
    Foo() : fd(0) { std::cout << "Foo::Foo() this=" << this << " fd=" << fd << std::endl; }

    Foo(int d) : fd(d) { std::cout << "Foo::Foo(int) this=" << this << " fd=" << d << std::endl; }

    ~Foo() { std::cout << "Foo::~Foo() this=" << this << " fd=" << fd << std::endl; }

    Foo(const Foo &other) {
        fd = other.fd;
        std::cout << "Foo::Foo(const Foo &) this=" << this << " fd=" << fd << std::endl;
    }

    Foo &operator=(const Foo &other) {
        fd = other.fd;
        std::cout << "Foo::Foo &operator=(const Foo &) this=" << this << " fd=" << fd << std::endl;
        return *this;
    }

#if __cplusplus >= 201103L
    Foo(Foo &&other) {
        fd = other.fd;
        other.fd = -1;
        std::cout << "Foo::Foo(Foo &&) this=" << this << " fd=" << fd << std::endl;
    }

    Foo &operator=(Foo &&other) {
        fd = other.fd;
        other.fd = -1;
        std::cout << "Foo::Foo &operator=(Foo &&) this=" << this << " fd=" << fd << std::endl;
        return *this;
    }
#endif

    int fd;
};

void test_constructor() {
    std::cout << std::endl << "// Foo f1(1);" << std::endl;
    Foo f1(1);

    std::cout << std::endl << "// Foo f2(f1);" << std::endl;
    Foo f2(f1);

    std::cout << std::endl << "// Foo f3 = f1;" << std::endl;
    Foo f3 = f1;

    std::cout << std::endl << "// f3 = f1;" << std::endl;
    f3 = f1;

#if __cplusplus >= 201103L
    std::cout << std::endl << "// Foo f4(std::move(f1));" << std::endl;
    Foo f4(std::move(f1));

    std::cout << std::endl << "// f4 = std::move(f1);" << std::endl;
    f4 = std::move(f1);
#endif

    std::cout << "\n";
}

int main() {
    std::cout << "TEST Constructor\n";
    test_constructor();
    std::cout << "\n";
}

输出如下：移动构造/移动赋值 函数为 C++11 增加特性，需要使用 std::move 来调用移动构造函数。

// Foo f1(1);
Foo::Foo(int) this=0x7ffd3fc2420c fd=1

// Foo f2(f1);
Foo::Foo(const Foo &) this=0x7ffd3fc24208 fd=1

// Foo f3 = f1;
Foo::Foo(const Foo &) this=0x7ffd3fc24204 fd=1

// f3 = f1;
Foo::Foo &operator=(const Foo &) this=0x7ffd3fc24204 fd=1

// Foo f4(std::move(f1));
Foo::Foo(Foo &&) this=0x7ffd3fc24200 fd=1

// f4 = std::move(f1);
Foo::Foo &operator=(Foo &&) this=0x7ffd3fc24200 fd=-1

Foo::~Foo() this=0x7ffd3fc24200 fd=-1
Foo::~Foo() this=0x7ffd3fc24204 fd=1
Foo::~Foo() this=0x7ffd3fc24208 fd=1
Foo::~Foo() this=0x7ffd3fc2420c fd=-1

布置 new

基于 布置new 表达式可以实现STL中各种容器的 emplace 操作，借助 clangd 看到其默认实现为 operator new

标签：__,std,场景,构造,fd,函数调用,Foo,type,构造函数
From： https://www.cnblogs.com/shuqin/p/18204068