所以我有这个代码：
class xx
{
int getnum(); //Is this a forward declaration or a prototype declaration and why?
};
int xx::getnum()
{
return 1+3;
}
所以这个问题已经在代码中注释了，但是：
Is int getnum();前向声明或原型声明，为什么？
C++ 标准中都没有定义术语“前向声明”和“原型声明”，所以严格来说，两者都不是。这简直就是一个宣言。如果你想更具体，你可以称之为非定义性声明，或“不是定义的声明”。
当标准中使用“前向声明”一词时，它们用于指声明但不定义它们声明的事物（函数或类）的声明。按照这种用法，int getnum()因此是一个前瞻性声明。
"Prototype declaration" of a function is used even less in the standard(1), and mostly when talking about [in]compatibility with C. However, when used, it refers to the exact same concept as a forward declaration of that function. Going by this, you could also call int getnum(); a prototype declaration.
综上所述，术语“前向声明”和“原型声明”没有正式的定义，但从它们通常使用和理解的方式来看，int getnum();可以描述为任一。
(1) However, a similar term "function prototype scope" is defined in the standard (C++17 N4659 [basic.scope.proto] 6.3.4/1). It establishes the scope of function parameter names in a non-defining function declaration, and it's the closest enclosing function declarator.
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