为什么在if条件中解析？ [英] Why vexing parse in an if condition?

问题描述

考虑代码：

#include <iostream>

struct Foo
{
    Foo(int){}
    operator bool() const
    {
        return true;
    }
};

int main()
{
    if(Foo foo{42})
    {
        std::cout << "ok\n";
    }
}

它在gcc5下编译得很好。但是，如果我替换 if（Foo foo {42}）与

It compiles fine under gcc5. However, if I replace the line if(Foo foo{42}) with

if(Foo foo(42))

我得到一个编译时错误： p>

I get a compile-time error:

错误：'foo'之前的主要表达式

error: expected primary-expression before 'foo'

这里发生了什么？

推荐答案

条件的语法不包括经典的构造函数调用。

The syntax for a condition does not include classic constructor invocation.

：

     attribute-specifier- seq _opt decl-specifier-seq declarator = initializer-clause

      attribute-specifier-seq _opt decl-specifier-seq declarator braced-init-list

condition:
    expression
    attribute-specifier-seq_opt decl-specifier-seq declarator = initializer-clause
    attribute-specifier-seq_opt decl-specifier-seq declarator braced-init-list

在中使用，开关， while 和 do 。我很惊讶，现在发现它用于 switch 。我从来没有想过这是一个条件。

This is used in if, switch, while and do. I was surprised to now discover that it's used in switch. I never thought of that as a condition.

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