C ++ 11：v = {}和v {}之间的差异 [英] C++11: Difference between v = { } and v { }

问题描述

我想询问以下两个语句之间是否有任何区别：

I would like to ask if there is any difference between following two statements:

// C++11
std::vector<int> d {1, 2, 3};
std::vector<int> d = {1, 2, 3};

在这两种情况下，都会调用序列构造函数：

In both cases the sequence constructor gets invoked:

class A {
public:
  int a;

  A() {
    cout << "default constructor" << endl;
  };

  A(const A& other) {
    cout << "copy constructor" << endl;
  };

  A& operator =(const A& other) {
    cout << "assignment operator" << endl;
  }

  A(std::initializer_list<int> e) {
    cout << "sequence constructor" << endl;
  };

  A& operator =(std::initializer_list<int> e) {
    cout << "initializer list assignment operator" << endl;
  }
};

int main(int argc, char** argv) {

  A a {1, 2}; // sequence constructor
  A b = {1, 2}; // sequence constructor
  b = {1, 2}; // initializer list assignment operator
}

推荐答案

例如

A myA{...};

将始终调用适当的构造函数。

will always invoke the appropriate constructor.

但是

A myA = ...;

将根据作业右侧的表达式执行不同的操作，调用复制构造函数。

will do different things depending on the expression on the right-hand side of the assignment, as it might invoke the copy-constructor instead. Or optimize it out with copy-elision.

此外，如果你有一个数组，你必须使用赋值语法。

Also, if you have an array you have to use the assignment syntax.

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