内联允许您在头文件中放置函数定义，并在多个源文件中#包含该头文件，而不违反一个定义规则。

在优化过程中，许多编译器会内联函数，即使你没有标记它们。如果你知道一些编译器不知道的东西，你通常只需要将函数标记为内联，因为它自己通常可以做出正确的决定。

优势

通过在需要的地方内联代码，程序将在函数调用和返回部分花费更少的时间。它应该使您的代码运行得更快，即使它变得更大(见下文)。内联普通访问器可以是有效内联的一个例子。 通过将其标记为内联，你可以将函数定义放在头文件中(即它可以包含在多个编译单元中，而链接器不会抱怨)

缺点

It can make your code larger (i.e. if you use inline for non-trivial functions). As such, it could provoke paging and defeat optimizations from the compiler. It slightly breaks your encapsulation because it exposes the internal of your object processing (but then, every "private" member would, too). This means you must not use inlining in a PImpl pattern. It slightly breaks your encapsulation 2: C++ inlining is resolved at compile time. Which means that should you change the code of the inlined function, you would need to recompile all the code using it to be sure it will be updated (for the same reason, I avoid default values for function parameters) When used in a header, it makes your header file larger, and thus, will dilute interesting informations (like the list of a class methods) with code the user don't care about (this is the reason that I declare inlined functions inside a class, but will define it in an header after the class body, and never inside the class body).

内联魔法

The compiler may or may not inline the functions you marked as inline; it may also decide to inline functions not marked as inline at compilation or linking time. Inline works like a copy/paste controlled by the compiler, which is quite different from a pre-processor macro: The macro will be forcibly inlined, will pollute all the namespaces and code, won't be easily debuggable, and will be done even if the compiler would have ruled it as inefficient. Every method of a class defined inside the body of the class itself is considered as "inlined" (even if the compiler can still decide to not inline it Virtual methods are not supposed to be inlinable. Still, sometimes, when the compiler can know for sure the type of the object (i.e. the object was declared and constructed inside the same function body), even a virtual function will be inlined because the compiler knows exactly the type of the object. Template methods/functions are not always inlined (their presence in an header will not make them automatically inline). The next step after "inline" is template metaprograming . I.e. By "inlining" your code at compile time, sometimes, the compiler can deduce the final result of a function... So a complex algorithm can sometimes be reduced to a kind of return 42 ; statement. This is for me extreme inlining. It happens rarely in real life, it makes compilation time longer, will not bloat your code, and will make your code faster. But like the grail, don't try to apply it everywhere because most processing cannot be resolved this way... Still, this is cool anyway...:-p

Generally speaking, these days with any modern compiler worrying about inlining anything is pretty much a waste of time. The compiler should actually optimize all of these considerations for you through its own analysis of the code and your specification of the optimization flags passed to the compiler. If you care about speed, tell the compiler to optimize for speed. If you care about space, tell the compiler to optimize for space. As another answer alluded to, a decent compiler will even inline automatically if it really makes sense.

另外，正如其他人所说，使用内联并不能保证任何东西都是内联的。如果你想要保证它，你必须定义一个宏而不是一个内联函数来实现它。

何时内联和/或定义宏强制包含?-只有当你对关键代码段的速度有了证明和必要的提高，并且知道这对应用程序的整体性能有影响时。

在古老的C和c++中，内联就像寄存器:给编译器一个关于可能的优化的建议(只不过是一个建议)。

在现代c++中，内联告诉链接器，如果在不同的翻译单元中发现了多个定义(不是声明)，那么它们都是相同的，链接器可以自由地保留其中一个，并丢弃所有其他的定义。

如果一个函数(无论多么复杂或“线性”)定义在头文件中，内联是强制的，以允许多个源包含它而不会被链接器产生“多个定义”错误。

默认情况下，类内部定义的成员函数是“内联”的，模板函数也是如此(与全局函数相反)。

//fileA.h
inline void afunc()
{ std::cout << "this is afunc" << std::endl; }

//file1.cpp
#include "fileA.h"
void acall()
{ afunc(); }

//main.cpp
#include "fileA.h"
void acall();

int main()
{ 
   afunc(); 
   acall();
}

//output
this is afunc
this is afunc

注意fileA.h包含在两个.cpp文件中，导致两个afunc()实例。 链接器将丢弃其中一个。 如果没有指定inline，链接器将报错。

内联允许您在头文件中放置函数定义，并在多个源文件中#包含该头文件，而不违反一个定义规则。