为什么gcc警告仅缩小统一初始化的转换范围？ [英] Why gcc warns about narrowing conversion only for uniform initialization?

问题描述

我正在尝试使用long 类型变量转换为 int 类型变量/en.cppreference.com/w/cpp/language/list_initialization rel = nofollow noreferrer> 统一初始化 ，并且没有它。但是我只有在统一初始化时才收到编译器警告。这是为什么？为什么在两种情况下 gcc 都不发出警告？我也尝试过使用 clang 并获得类似的结果。

I am trying to convert long type variable to int type variable with uniform initialization and without it. But I get compiler warning only with uniform initialization. Why is that? Why does not gcc warn in both cases? I have tried with clang also and got similar results.

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;
}

这是我得到的唯一警告

$ 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 };

推荐答案

因为标准说，缩小转换限制仅用于列表初始化（自C ++ 11起）。

Because the standard says, narrowing conversions limit is specified only for list initialization (since C++11).

列表初始化限制
允许的隐式转换，禁止以下操作：

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

• 从浮点类型转换为整数类型


• 从长整型转换为double或double，然后从double转换为float，除非source是一个常量表达式
  ，不会发生溢出


• 从整数类型到浮点类型的转换，除非source是一个可以存储值的常量表达式
  完全符合目标类型


• 从整数或无作用域枚举类型到无法表示的整数类型的转换nt原始的所有值，除非源
  是一个常数表达式，其值可以精确地存储在
  目标类型中

• 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），不应用缩小转换限制规则（添加）；因为那样会破坏很多旧代码。

For the other initialization methods (using parentheses or equal sign), narrowing conversions limit rule is not applied (added); because that might break much legacy code.

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