I do not understand why this compiles

I'm certainly missing something, but I do not understand why this compiles (with both g++ & clang++):

struct A
{
};
struct B
{
};


int main()
{
A a(B);
}

First of all, B is a type... not a value. How should I interpret this code?

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最佳答案

It's interpreted as the declaration of a function named a, which takes one argument of type B and returns A.

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It's simply a function declaration declaring a to be a function returning A and taking one unnamed parameter of type B.

It is valid because function declarations as opposed to function definitions are allowed within function definitions.

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This issue is known as the most vexing parse. The line A a(B); can be interpreted as the declaration of a function named a returning an object of type A and taking an unnamed parameter of type B.

One way to avoid this issue is to use the uniform initialization syntax which was introduced in C++11, which consists in using braces instead of parenthesis: A a{B}; returns an error. The line is now interpreted as a variable declaration initialized with B, which is a type instead of a value.

Here's more information:

The Most Vexing Parse: How to Spot It and Fix It Quickly


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