为什么 Foo(b) 在 C++ 中编译成功？

Question

当我编译下面的代码时有一个奇怪的行为：

class Foo {
};
int main() {
    Foo(b);
}

即使不声明b也能编译成功。有什么解释吗？

Answer 1

它本身就是一个声明。它声明了一个名为 b 且类型为 Foo 的变量，即与 Foo b;.

相同的效果

[stmt.ambig]/1

There is an ambiguity in the grammar involving expression-statements and declarations: An expression-statement with a function-style explicit type conversion as its leftmost subexpression can be indistinguishable from a declaration where the first declarator starts with a (. In those cases the statement is a declaration.

和[stmt.ambig]/2

The remaining cases are declarations. [ Example:
class T {
  // ...
public:
  T();
  T(int);
  T(int, int);
};
T(a);               //  declaration
...

为什么 Foo(b) 在 C++ 中编译成功？

Why does Foo(b) compile successfully in C++?

c++

declaration

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