为什么仅在列表初始化的情况下才会出现缩小转换警告? [英] Why does a narrowing conversion warning appear only in case of list initialization?

问题描述

我有以下代码:

class A
{
    public:
        A(const unsigned int val) : value(val) {}

        unsigned int value;
};

int main()
{
    int val = 42;
    A a(val);
    A b{val};       // <--- Warning in GCC, error in Microsoft Visual Studio 2015

    return 0;
}

为什么仅在使用列表初始化的情况下才会出现缩小转换警告?

Why does the narrowing conversion warning appear only in case of list initialization usage?

推荐答案

列表初始化自C ++ 11开始引入，其功能禁止在内置类型之间进行隐式缩小转换.同时，另外两个使用括号和等号的旧式"(自C ++ 98起)初始化形式

list initialization was introduced since C++11 with the feature prohibiting implicit narrowing conversions among built-in types. At the same time, the other two "old-style" (since C++98) initialization forms which use parentheses and equal-sign like

int val = 42;
A a(val);
A a = val;

请勿更改其行为以符合列表初始化，因为这可能会破坏大量的旧代码库.

don't change their behavior to accord with list initialization, because that could break plenty of legacy code bases.

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