c++ - 为什么 gcc 仅针对统一初始化警告缩小转换？

Question

我正在尝试将 long 类型变量转换为 int 类型变量 uniform initialization没有它。但是我只在统一初始化时收到编译器警告。这是为什么？为什么 gcc 在这两种情况下都不发出警告？我也尝试过使用 clang 并得到了类似的结果。

这是代码

#include <iostream>

int main() {
    long l = 1;
    int i1 = l;
    int i2 = { l };

    std::cout << i1 << std::endl;
    std::cout << i2 << std::endl;

    return 0;
}

我收到的唯一一个警告

$ g++ -Wall -Wextra 1.cpp
1.cpp: In function ‘int main()’:
1.cpp:6:16: warning: narrowing conversion of ‘l’ from ‘long int’ to ‘int’ inside { } [-Wnarrowing]
   int i2 = { l };

Answer 1

因为标准说，narrowing conversions limit仅指定用于列表初始化(C++11 起)。

list-initialization limits the allowed implicit conversions by prohibiting the following:

• conversion from a floating-point type to an integer type
• conversion from a long double to double or to float and conversion from double to float, except where the source is a constant expression and overflow does not occur
• conversion from an integer type to a floating-point type, except where the source is a constant expression whose value can be stored exactly in the target type
• conversion from integer or unscoped enumeration type to integer type that cannot represent all values of the original, except where source is a constant expression whose value can be stored exactly in the target type

对于其他初始化方法(使用括号或等号)，不应用(添加)缩小转换限制规则；因为这可能会破坏很多遗留代码。