我想补充一点，在构建共享库时，内联函数是至关重要的。如果不将函数标记为内联，则它将以二进制形式导出到库中。如果导出，它也将出现在符号表中。另一方面，内联函数不会被导出，既不会被导出到库二进制文件中，也不会被导出到符号表中。

当库打算在运行时加载时，它可能是关键的。它还可能打击二进制兼容的库。在这种情况下，不要使用内联。

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。

我想补充一点，在构建共享库时，内联函数是至关重要的。如果不将函数标记为内联，则它将以二进制形式导出到库中。如果导出，它也将出现在符号表中。另一方面，内联函数不会被导出，既不会被导出到库二进制文件中，也不会被导出到符号表中。

当库打算在运行时加载时，它可能是关键的。它还可能打击二进制兼容的库。在这种情况下，不要使用内联。

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

内联是对编译器的一个建议，它可以随意忽略。它非常适合小段代码。

如果你的函数是内联的，它基本上是插入到函数调用的代码中，而不是实际调用一个单独的函数。这有助于提高速度，因为你不必实际打电话。

它还帮助cpu进行流水线操作，因为它们不必用调用引起的新指令重新加载流水线。

唯一的缺点是可能会增加二进制大小，但只要函数很小，这就不会太大。

现在我倾向于把这类决定留给编译器(好吧，至少是聪明的编译器)。编写它们的人往往对底层架构有更详细的了解。

这并不完全是关于性能。c++和C都用于嵌入式编程，位于硬件之上。例如，如果您要编写中断处理程序，则需要确保代码可以立即执行，而不会交换额外的寄存器和/或内存页。这就是内联派上用场的时候。好的编译器在需要速度时自己做一些“内联”，但“内联”迫使它们这样做。