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Copy pathmove.cpp
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290 lines (277 loc) · 8.56 KB
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//
// Created by william on 2021/3/28.
//
#include "base.h"
#include <utility>
#include <vector>
/// 可作为面试题：可参考链接：https://segmentfault.com/q/1010000008984307
/// move的语义是经由移动构造函数创建的副本，本质上是强转，如果没有移动构造函数，还是会走拷贝构造函数。
class StringTest
{
public:
StringTest()
{
std::cout << "constructor" << std::endl;
}
explicit StringTest(std::string s) :
m_s(std::move(s))
{
std::cout << "constructor()" << std::endl;
}
~StringTest()
{
std::cout << "distructor" << std::endl;
}
StringTest(const StringTest&)
{
std::cout << "copy constructor" << std::endl;
}
StringTest(const StringTest&&) noexcept
{
std::cout << "move constructor" << std::endl;
}
private:
[[maybe_unused]] std::string m_s;
};
[[maybe_unused]] void moveTest()
{
std::string str = "hello";
std::vector<std::string> v;
v.push_back(str);
// 使用push_back(const T&)重载
// 表示我们将带来复制str的成本
std::cout << "After copy, str is \"" << str << "\"\n";
// 使用右值引用 push_back(T&&)重载
// 表示不复制字符串，而是
// str的内容被移动进vector
// 这个开销可能比较低，但是也意味着str现在可能为空
v.emplace_back(std::move(str));
std::cout << "After move, str is \"" << str << "\"\n";
std::cout << "The contents of the vector are \"" << v[0]
<< "\", \"" << v[1] << "\"\n";
StringTest test;
StringTest test2;
std::vector<StringTest> v1;
std::cout << "1========v1.push_back(test)=========" << std::endl;
v1.push_back(test);
std::cout << "1========v1.emplace_back(test)=========" << std::endl;
v1.emplace_back(test);
std::cout << "1========v1.push_back(std::move(test))" << std::endl;
v1.push_back(std::move(test));
std::cout << "1========v1.emplace_back(test2)=========" << std::endl;
v1.emplace_back(test2);
std::cout << "1========v1.emplace_back(std::move(test2))" << std::endl;
v1.emplace_back(std::move(test2));
std::cout << "1========v1.emplace_back(\"hello\")=========" << std::endl;
v1.emplace_back("hello");
/// 注意v1.reserve(10);这一句很关键，vector扩容，一开始容量是0，第一次操作扩容到1，第二次是翻倍为2，会造成拷贝
/// 所以在你执行第一次push_back时，检查了容量不够，执行__push_back_slow_path函数，vector扩容到1，并且执行了一次移动构造函数，因为原来的vector是空的，所以不需要进一步处理。
/// 此时第一次打印 copy constructor
/// 在执行第二次emplace_back时，检查了容量不够，执行__push_back_slow_path函数，vector扩容到2，并且执行了一次拷贝构造函数。
v1.reserve(10);
std::cout << "2========v1.push_back(test)=========" << std::endl;
v1.push_back(test);
std::cout << "2========v1.emplace_back(test)=========" << std::endl;
v1.emplace_back(test);
std::cout << "2========v1.push_back(std::move(test))::good=========" << std::endl;
v1.push_back(std::move(test));
std::cout << "2========v1.emplace_back(test2)=========" << std::endl;
v1.emplace_back(test2);
std::cout << "2========v1.emplace_back(std::move(test2))::good=========" << std::endl;
v1.emplace_back(std::move(test2));
std::cout << "2========v1.emplace_back(\"hello\")=========" << std::endl;
v1.emplace_back("hello");
std::cout << "2========end=========" << std::endl;
/// 需要指的注意的是：在代码36行有没有加noexcept区别是很大的,这是是没加noexcept之前的结果
/**
* /Users/william/git/sample/cpp_demo/cmake-build-debug/cpp_demo
* After copy, str is "hello"
* After move, str is ""
* The contents of the vector are "hello", "hello"
* constructor
* constructor
* 1========v1.push_back(test)=========
* copy constructor
* 1========v1.emplace_back(test)=========
* copy constructor
* copy constructor
* distructor
* 1========v1.push_back(std::move(test))
* move constructor
* copy constructor
* copy constructor
* distructor
* distructor
* 1========v1.emplace_back(test2)=========
* copy constructor
* 1========v1.emplace_back(std::move(test2))
* move constructor
* copy constructor
* copy constructor
* copy constructor
* copy constructor
* distructor
* distructor
* distructor
* distructor
* 1========v1.emplace_back("hello")=========
* constructor()
* copy constructor
* copy constructor
* copy constructor
* copy constructor
* copy constructor
* copy constructor
* distructor
* distructor
* distructor
* distructor
* distructor
* distructor
* 2========v1.push_back(test)=========
* copy constructor
* 2========v1.emplace_back(test)=========
* copy constructor
* 2========v1.push_back(std::move(test))::good=========
* move constructor
* 2========v1.emplace_back(test2)=========
* copy constructor
* 2========v1.emplace_back(std::move(test2))::good=========
* move constructor
* copy constructor
* copy constructor
* copy constructor
* copy constructor
* copy constructor
* copy constructor
* copy constructor
* copy constructor
* copy constructor
* copy constructor
* distructor
* distructor
* distructor
* distructor
* distructor
* distructor
* distructor
* distructor
* distructor
* distructor
* 2========v1.emplace_back("hello")=========
* constructor()
* 2========end=========
* distructor
* distructor
* distructor
* distructor
* distructor
* distructor
* distructor
* distructor
* distructor
* distructor
* distructor
* distructor
* distructor
* distructor
* Process finished with exit code 0
*/
/// 这是加了noexcept之后的结果，对比可以通过beyong compare对比一下,会发现move构造加了noexcept之后vector扩容打印出来的才是move构造
/**
*
* /Users/william/git/sample/cpp_demo/cmake-build-debug/cpp_demo
* After copy, str is "hello"
* After move, str is ""
* The contents of the vector are "hello", "hello"
* constructor
* constructor
* 1========v1.push_back(test)=========
* copy constructor
* 1========v1.emplace_back(test)=========
* copy constructor
* move constructor
* distructor
* 1========v1.push_back(std::move(test))
* move constructor
* move constructor
* move constructor
* distructor
* distructor
* 1========v1.emplace_back(test2)=========
* copy constructor
* 1========v1.emplace_back(std::move(test2))
* move constructor
* move constructor
* move constructor
* move constructor
* move constructor
* distructor
* distructor
* distructor
* distructor
* 1========v1.emplace_back("hello")=========
* constructor()
* move constructor
* move constructor
* move constructor
* move constructor
* move constructor
* move constructor
* distructor
* distructor
* distructor
* distructor
* distructor
* distructor
* 2========v1.push_back(test)=========
* copy constructor
* 2========v1.emplace_back(test)=========
* copy constructor
* 2========v1.push_back(std::move(test))::good=========
* move constructor
* 2========v1.emplace_back(test2)=========
* copy constructor
* 2========v1.emplace_back(std::move(test2))::good=========
* move constructor
* move constructor
* move constructor
* move constructor
* move constructor
* move constructor
* move constructor
* move constructor
* move constructor
* move constructor
* move constructor
* distructor
* distructor
* distructor
* distructor
* distructor
* distructor
* distructor
* distructor
* distructor
* distructor
* 2========v1.emplace_back("hello")=========
* constructor()
* 2========end=========
* distructor
* distructor
* distructor
* distructor
* distructor
* distructor
* distructor
* distructor
* distructor
* distructor
* distructor
* distructor
* distructor
* distructor
* Process finished with exit code 0
*
*/
}