为什么是“ Foo f（Bar（））;”？可以是采用Bar类型并返回Foo类型的函数的声明？ [英] Why "Foo f(Bar());" can be a declaration of a function that takes type Bar and returns type Foo?

问题描述

我遇到了这个C ++问题：

I met this C++ question:

问题：以下是定义还是声明？

Question: Is the following a definition or a declaration?

Foo f(Bar());

答案：可能是函数声明为Bar类型并返回Foo类型，或者是 f 的定义，类型为 Foo ，其构造函数采用Bar类型。问题是两者的语法都相同，因此要解决此问题，C ++标准指出，在无法区分的情况下，编译器必须更喜欢函数声明而不是对象定义。

Answer: It is possibly either a declaration of a function that takes type Bar and returns type Foo or it is a definition of f as a type Foo, which has a constructor that takes type Bar. The problem is the syntax for both is identical so to resolve this problem the C++ standard states that a compiler must prefer function declarations to object definitions where it is unable to make a distinction.

-我不明白为什么它可以是声明Bar类型并返回Foo类型的函数的声明？括号（）为何出现在参数列表中？

-- I don't understand why it can be "a declaration of a function that takes type Bar and returns type Foo"? how come a parenthesis "()" appear in parameter list?

推荐答案

函数 f 实际上将一个函数指针指向一个不带参数的函数，并给出一个 Bar 。 f 的参数类型为 Bar（*）（）。

The function f actually takes a function pointer to a function that takes no arguments and gives a Bar. The type of the argument to f is Bar (*)().

此代码无法编译（我们可以在错误消息中看到参数的实际类型）：

This code fails to compile (and we can see the actual type of the argument in the error message):

class Foo { };
class Bar { };

Foo f(Bar());

int main() {
  Bar b;
  f(b);
  return 0;
}

但是此代码可以编译：

class Foo { };
class Bar { };

Foo f(Bar());

Bar g();

int main() {
  f(g);
  return 0;
}

正如您在问题中所说，它的第二个含义是您正在创建一个名为 f 的新的 Foo 对象，并使用 Bar（ ）（ Bar 的新实例）。类似于：

The second meaning it could have, as you say in the question, is that you are making a new Foo object called f and you are calling the constructor with Bar() (a new instance of Bar). It would be similar to:

Foo f = Foo(Bar());

在这种情况下 Foo f（Bar（））;

In this situation of Foo f(Bar()); though, the first interpretation is chosen by the compiler.

有些令人困惑的是，如果您添加另一组括号，例如

Somewhat confusingly, if you add another set of parentheses, as in

Foo f((Bar()));

编译器选择第二种解释。

the compiler picks the second interpretation.

这篇关于为什么是“ Foo f（Bar（））;”？可以是采用Bar类型并返回Foo类型的函数的声明？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！