在c++中使用内联函数的优点/缺点是什么?我看到它只提高了编译器输出的代码的性能,但随着今天优化的编译器,快速的cpu,巨大的内存等(不像在1980年<内存是稀缺的,所有东西都必须适合100KB内存),他们今天真正有什么优势?
当前回答
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。
其他回答
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
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。
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。
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.
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。