为什么列表初始化（使用花括号）比其他选项更好？

Question

MyClass a1 {a};     // clearer and less error-prone than the other three
MyClass a2 = {a};
MyClass a3 = a;
MyClass a4(a);

为什么？

Answer 1

基本上是从Bjarne Stroustrup的“C++编程语言第四版”中复制和粘贴的：

列表初始化不允许缩小(§ISO.8.5.4)。即：

• 一个整数不能转换为另一个不能容纳其值的整数。例如，允许char to int，但不允许int to char。
• 浮点值不能转换为无法保存其值的其他浮点类型。例如，允许float to double，但不允许double to float。
• 浮点值不能转换为整数类型。
• 整数值不能转换为浮点类型。

void fun(double val, int val2) {

    int x2 = val;    // if val == 7.9, x2 becomes 7 (bad)

    char c2 = val2;  // if val2 == 1025, c2 becomes 1 (bad)

    int x3 {val};    // error: possible truncation (good)

    char c3 {val2};  // error: possible narrowing (good)

    char c4 {24};    // OK: 24 can be represented exactly as a char (good)

    char c5 {264};   // error (assuming 8-bit chars): 264 cannot be 
                     // represented as a char (good)

    int x4 {2.0};    // error: no double to int value conversion (good)

}

auto z1 {99};   // z1 is an int
auto z2 = {99}; // z2 is std::initializer_list<int>
auto z3 = 99;   // z3 is an int