在C ++中，我是否要为自己不吃的东西付费？ [英] In C++, am I paying for what I am not eating?

问题描述

让我们考虑以下用C和C ++编写的世界示例：

Let's consider the following hello world examples in C and C++:

main.c

#include <stdio.h>

int main()
{
    printf("Hello world\n");
    return 0;
}

main.cpp

#include <iostream>

int main()
{
    std::cout<<"Hello world"<<std::endl;
    return 0;
}

当我将它们编译为汇编时，C代码的大小为仅9行（ gcc -O3 ）：

When I compile them in godbolt to assembly, the size of the C code is only 9 lines (gcc -O3):

.LC0:
        .string "Hello world"
main:
        sub     rsp, 8
        mov     edi, OFFSET FLAT:.LC0
        call    puts
        xor     eax, eax
        add     rsp, 8
        ret

但是C ++代码的大小是22行（ g ++ -O3 ）：

But the size of the C++ code is 22 lines (g++ -O3):

.LC0:
        .string "Hello world"
main:
        sub     rsp, 8
        mov     edx, 11
        mov     esi, OFFSET FLAT:.LC0
        mov     edi, OFFSET FLAT:_ZSt4cout
        call    std::basic_ostream<char, std::char_traits<char> >& std::__ostream_insert<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*, long)
        mov     edi, OFFSET FLAT:_ZSt4cout
        call    std::basic_ostream<char, std::char_traits<char> >& std::endl<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&)
        xor     eax, eax
        add     rsp, 8
        ret
_GLOBAL__sub_I_main:
        sub     rsp, 8
        mov     edi, OFFSET FLAT:_ZStL8__ioinit
        call    std::ios_base::Init::Init() [complete object constructor]
        mov     edx, OFFSET FLAT:__dso_handle
        mov     esi, OFFSET FLAT:_ZStL8__ioinit
        mov     edi, OFFSET FLAT:_ZNSt8ios_base4InitD1Ev
        add     rsp, 8
        jmp     __cxa_atexit

...这要大得多。

在C ++中，您为自己所付的钱而闻名吃。因此，在这种情况下，我要支付的费用是什么？

It is famous that in C++ you pay for what you eat. So, in this case, what am I paying for?

推荐答案

您要支付的费用是调用繁重的库（不是和打印到控制台一样重）。您初始化 ostream 对象。有一些隐藏的存储。然后，您调用 std :: endl ，它不是 \n 的同义词。 iostream 库可帮助您调整许多设置，并减轻处理器而不是程序员的负担。这就是您要支付的费用。

What you are paying for is to call a heavy library (not as heavy as printing into console). You initialize an ostream object. There are some hidden storage. Then, you call std::endl which is not a synonym for \n. The iostream library helps you adjusting many settings and putting the burden on the processor rather than the programmer. This is what you are paying for.

让我们检查一下代码：

.LC0:
        .string "Hello world"
main:

初始化ostream对象+ cout

Initializing an ostream object + cout

    sub     rsp, 8
    mov     edx, 11
    mov     esi, OFFSET FLAT:.LC0
    mov     edi, OFFSET FLAT:_ZSt4cout
    call    std::basic_ostream<char, std::char_traits<char> >& std::__ostream_insert<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*, long)

再次调用 cout 打印新行并刷新

Calling cout again to print a new line and flush

    mov     edi, OFFSET FLAT:_ZSt4cout
    call    std::basic_ostream<char, std::char_traits<char> >& std::endl<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&)
    xor     eax, eax
    add     rsp, 8
    ret

静态存储初始化：

_GLOBAL__sub_I_main:
        sub     rsp, 8
        mov     edi, OFFSET FLAT:_ZStL8__ioinit
        call    std::ios_base::Init::Init() [complete object constructor]
        mov     edx, OFFSET FLAT:__dso_handle
        mov     esi, OFFSET FLAT:_ZStL8__ioinit
        mov     edi, OFFSET FLAT:_ZNSt8ios_base4InitD1Ev
        add     rsp, 8
        jmp     __cxa_atexit

此外，区分语言和库也是必不可少的。

Also, it is essential to distinguish between the language and the library.

BTW，这只是故事的一部分。您不知道正在调用的函数中写了什么。

BTW, this is just a part of the story. You do not know what is written in the functions you are calling.

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