为什么 g++使我的代码以与编写的顺序不同的顺序执行，如何禁用此“优化”？

发布于 2024-12-28 13:46:00 字数 341 浏览 3 评论 0原文

例如：

#include <stdio.h>
#include <string>
int main() {
    std::string* stuff(NULL);

    printf("allocating memory..."); //line 2

    stuff = new std::string[500000000]; //line 3

    delete [] stuff; //line 4

    return 0;
}

执行时在第 2 行之前运行第 3 行（也可能是第 4 行）。现在我知道这可能是一些很好的优化功能，但有时需要正确的顺序。

原文

For example:

#include <stdio.h>
#include <string>
int main() {
    std::string* stuff(NULL);

    printf("allocating memory..."); //line 2

    stuff = new std::string[500000000]; //line 3

    delete [] stuff; //line 4

    return 0;
}

when executed runs line 3 (and possibly line 4) before line 2. Now I know this is probably some good optimization feature but sometimes the right order is needed.

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一向肩并 2025-01-04 13:46:00

问题就在这里：

printf("allocating memory..."); //line 2

在许多架构中，您都有缓冲输出，这意味着您在屏幕上打印的内容不会立即显示，而是存储在内存缓冲区中。要刷新缓冲区并确保立即打印它，您可以使用

printf("allocating memory...\n"); //line 2 with the \n character that flushes the buffer

尽管除了个人经验之外我没有找到任何东西可以证明这一点，或者，如果您不想换行（并且绝对确定刷新），您可以在第 2 行之后使用 fflush(stdout) 。

The problem is here:

printf("allocating memory..."); //line 2

In many architectures you have buffered output, which means that what you print on the screen is not shown immediately but stored in a memory buffer. To flush the buffer and ensure that he is printed immediately, you can use

printf("allocating memory...\n"); //line 2 with the \n character that flushes the buffer

although I didn't find anything to prove this besides personal experience, or alternatively, if you don't want to go to a new line (and be absolutely sure of flushing) you can use fflush(stdout) right after line 2.

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卖梦商人 2025-01-04 13:46:00

在 C++ 中，你可能想这样写：

#include <iostream>
#include <string>
int main() {
    std::string* stuff(NULL);

    std::cout << "allocating memory..."; //line 2
    std::cout.flush();   // ensures the buffer is actually written to the terminal right here

    stuff = new std::string[500000000]; //line 3

    delete [] stuff; //line 4

    return 0;
}

In C++, you might want to write it like this:

#include <iostream>
#include <string>
int main() {
    std::string* stuff(NULL);

    std::cout << "allocating memory..."; //line 2
    std::cout.flush();   // ensures the buffer is actually written to the terminal right here

    stuff = new std::string[500000000]; //line 3

    delete [] stuff; //line 4

    return 0;
}

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