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

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。

其他回答

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);

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。

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。

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.