c++ - 函数到函数指针 "decay"

Question

我们知道一个看起来像 void() 的参数将被重写为 void(*)()。这类似于数组到指针的衰减，其中 int[] 变为 int*。在很多情况下，使用数组会将其衰减为指针。除了参数之外，还有没有函数“衰减”的情况？

C++ 标准规定:

§8.3.5/5

... After determining the type of each parameter, any parameter of type “array of T” or “function returning T” is adjusted to be “pointer to T” or “pointer to function returning T,” respectively...

由于下面的评论者似乎不相信我..这是我的编译器显示的内容。

void handler(void func())
{
    func(42);
}

main.cpp: In function 'void handler(void (*)())':
main.cpp:5:12: error: too many arguments to function
     func(42);
        ^

Answer 1

存在三种被视为左值转换的转换:左值到右值、数组到指针和函数到指针。你可以称之为“衰变”，因为这就是 std::decay可以处理这些类型，但标准只是将其称为函数到指针的转换 [conv.func]:

An lvalue of function type T can be converted to a prvalue of type “pointer to T.” The result is a pointer to the function.

如果您要询问发生函数到指针转换的情况，它们与其他两个左值转换发生时的情况基本相同。如果我们只是按顺序浏览标准，以下是发生函数到指针转换的详尽案例列表:

使用函数作为操作数，[expr]/9:

Whenever a glvalue expression appears as an operand of an operator that expects a prvalue for that operand, the lvalue-to-rvalue (4.1), array-to-pointer (4.2), or function-to-pointer (4.3) standard conversions are applied to convert the expression to a prvalue.

使用函数作为可变参数函数的参数，[expr.call]/7:

When there is no parameter for a given argument, the argument is passed in such a way that the receiving function can obtain the value of the argument by invoking va_arg (18.10)... The lvalue-to-rvalue (4.1), array-to-pointer (4.2), and function-to-pointer (4.3) standard conversions are performed on the argument expression.

您可以static_cast去掉这个转换，[expr.static.cast]/7:

The inverse of any standard conversion sequence (Clause 4) not containing an lvalue-to-rvalue (4.1), arrayto- pointer (4.2), function-to-pointer (4.3), null pointer (4.10), null member pointer (4.11), or boolean (4.12) conversion, can be performed explicitly using static_cast.

否则，你传入的操作数会被转换，[expr.static.cast]/8:

The lvalue-to-rvalue (4.1), array-to-pointer (4.2), and function-to-pointer (4.3) conversions are applied to the operand.

使用 reinterpret_cast , [expr.reinterpret.cast]/1:

The result of the expression reinterpret_cast<T>(v) is the result of converting the expression v to type T. If T is an lvalue reference type or an rvalue reference to function type, the result is an lvalue; if T is an rvalue reference to object type, the result is an xvalue; otherwise, the result is a prvalue and the lvalue-torvalue (4.1), array-to-pointer (4.2), and function-to-pointer (4.3) standard conversions are performed on the expression v.

使用 const_cast , [expr.const.cast]，与上面的措辞基本相同。使用条件运算符，[expr.cond]:

Lvalue-to-rvalue (4.1), array-to-pointer (4.2), and function-to-pointer (4.3) standard conversions are performed on the second and third operands.

请注意，在上述所有情况下，它总是所有的左值转换。

在模板中也会发生函数到指针的转换。将函数作为非类型参数传递，[temp.arg.nontype]/5.4:

For a non-type template-parameter of type pointer to function, the function-to-pointer conversion (4.3) is applied

或者键入deduction，[temp.deduct.call]/2:

If P is not a reference type:

• — If A is an array type, the pointer type produced by the array-to-pointer standard conversion (4.2) is used in place of A for type deduction; otherwise,
• — If A is a function type, the pointer type produced by the function-to-pointer standard conversion (4.3) is used in place of A for type deduction; otherwise,

或转换函数模板推演，写法大致相同。

最后当然是std::decay本身，在 [meta.trans.other] 中定义，强调我的:

Let U be remove_reference_t<T>. If is_array<U>::value is true, the member typedef type shall equal remove_extent_t<U>*. If is_function<U>::value is true, the member typedef type shall equal add_pointer_t<U>. Otherwise the member typedef type equals remove_cv_t<U>. [ Note: This behavior is similar to the lvalue-to-rvalue (4.1), array-to-pointer (4.2), and function-to-pointer (4.3) conversions applied when an lvalue expression is used as an rvalue, but also strips cv-qualifiers from class types in order to more closely model by-value argument passing. —end note ]