内联函数更快，因为你不需要将参数和返回地址等东西推入或弹出堆栈;但是，它确实会使二进制文件略大一些。

有显著的区别吗?对大多数人来说，在现代硬件上还不够明显。但它可以产生影响，这对一些人来说已经足够了。

将某些东西标记为内联并不能保证它将是内联的。这只是给编译器的一个建议。有时候是不可能的比如你有一个虚函数，或者涉及到递归。有时候编译器会选择不使用它。

我可以看到这样的情况会产生明显的不同:

inline int aplusb_pow2(int a, int b) {
  return (a + b)*(a + b) ;
}

for(int a = 0; a < 900000; ++a)
    for(int b = 0; b < 900000; ++b)
        aplusb_pow2(a, b);

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。

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

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。

另一个讨论的结论是:

内联函数有什么缺点吗?

显然，使用内联函数并没有什么错。

但值得注意的是以下几点!

Overuse of inlining can actually make programs slower. Depending on a function's size, inlining it can cause the code size to increase or decrease. Inlining a very small accessor function will usually decrease code size while inlining a very large function can dramatically increase code size. On modern processors smaller code usually runs faster due to better use of the instruction cache. - Google Guidelines The speed benefits of inline functions tend to diminish as the function grows in size. At some point the overhead of the function call becomes small compared to the execution of the function body, and the benefit is lost - Source There are few situations where an inline function may not work: For a function returning values; if a return statement exists. For a function not returning any values; if a loop, switch or goto statement exists. If a function is recursive. -Source The __inline keyword causes a function to be inlined only if you specify the optimize option. If optimize is specified, whether or not __inline is honored depends on the setting of the inline optimizer option. By default, the inline option is in effect whenever the optimizer is run. If you specify optimize , you must also specify the noinline option if you want the __inline keyword to be ignored. -Source

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。

内联函数是编译器使用的优化技术。可以简单地在函数原型前加上inline关键字来使函数内联。内联函数指示编译器在代码中使用函数的任何地方插入完整的函数体。

优点:-

It does not require function calling overhead. It also save overhead of variables push/pop on the stack, while function calling. It also save overhead of return call from a function. It increases locality of reference by utilizing instruction cache. After in-lining compiler can also apply intra-procedural optimization if specified. This is the most important one, in this way compiler can now focus on dead code elimination, can give more stress on branch prediction, induction variable elimination etc..

要了解更多信息，可以点击这个链接 http://tajendrasengar.blogspot.com/2010/03/what-is-inline-function-in-cc.html

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。

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。

在将函数内联到so库时遇到了同样的麻烦。似乎内联函数没有编译到库中。因此，如果一个可执行文件想要使用库的内联函数，链接器会输出一个“未定义引用”错误。(我碰巧用gcc 4.5编译Qt源代码。

Why not make all functions inline by default? Because it's an engineering trade off. There are at least two types of "optimization": speeding up the program and reducing the size (memory footprint) of the program. Inlining generally speeds things up. It gets rid of the function call overhead, avoiding pushing then pulling parameters from the stack. However, it also makes the memory footprint of the program bigger, because every function call must now be replaced with the full code of the function. To make things even more complicated, remember that the CPU stores frequently used chunks of memory in a cache on the CPU for ultra-rapid access. If you make the program's memory image big enough, your program won't be able to use the cache efficiently, and in the worst case inlining could actually slow your program down. To some extent the compiler can calculate what the trade offs are, and may be able to make better decisions than you can, just looking at the source code.

我们的计算机科学教授敦促我们不要在c++程序中使用内联。当被问及原因时，他友好地向我们解释说，现代编译器应该自动检测何时使用内联。

是的，内联可以是一种优化技术，在任何可能的地方都可以使用，但显然这已经为你做了，只要有可能内联一个函数。