可以“首次使用时构建”成语在任何情况下都会失败？ [英] Can "construct on first use" idiom fail under any circumstances?

问题描述

我正在使用一些静态库建立我的程序（测试）。

这个库包含一个文件，其中有我的功能：

I'm building my program (tests actually) using some static library.
This library contains one file inside which I have functions like that:

string& GetString() {
    static string strFilename;
    return strFilename;
}

void PrintToScreen() {
    printf("String: %s\n", GetString().c_str())
}

然后在我的main.cpp（库外）中：

Then in my main.cpp (outside the library) I'm doing:

GetString() = "abc";
printf("String: %s\n", GetString().c_str());
PrintToScreen();

我得到这个输出：

String: abc
String:

第二次调用函数
（但是从不同的文件，这是在库内部）
某种方式清除以前的值，重新初始化它，或使用它的自己的副本。

I改变GetString函数使用'new'，但结果是完全一样的（btw。程序从不崩溃）。

但是我不明白热的可能吗？

任何想法我错了什么？

So looks like second call to the function (but done from different file, which is inside the library) somehow clear previous value, reinitialize it, or uses own copy of it.
I changed GetString function to use 'new' but result is exactly the same (btw. program never crash).
But I don't understand hot it's possible?
Any ideas what I'm doing wrong?

-------------------- ----------- UPDATE ------------------------------则

------------------------------- UPDATE ------------------------------

1. 测试是单线程环境。


2. 它在某些平台上工作，


3. 我在执行过程中验证了strFilename的地址（printf在GetString中），以及看起来像是一个没有重复的变量（地址总是相同的）


4. 但是，在最后的lib我得到类似的东西：

1. Test is done is single threaded environment.
2. It works on some platforms and on some it doesn't (works on windows, MacOS and AIX, doesn't work on linux, HP_UX, Solaris, FreeBSD...)
3. I verified address of the strFilename during the execution (printf inside GetString) and looks like it's one variable without duplicates (address is always the same)
4. BUT, with nm on the final lib I get something like that:

0000000000000030 T _Z16GetLogprintfFilevogle
0000000000000008 b _ZGVZ16GetLogprintfFilevE16strLogprintfFileogle
0000000000000018 b_ZZ16GetLogprintfFilevE16strLogprintfFileã
U _Z16GetLogprintfFilev

并在我的基础库上使用nm（由最终lib使用）我得到：

and with nm on my base lib (used by final lib) I get:

0000000000000030 T _Z16GetLogprintfFilevogle
0000000000000008 b_ZGVZ16GetLogprintfFilevE16strLogprintfFileã
0000000000000018 b_ZZ16GetLogprintfFilevE16strLogprintfFileã

推荐答案

实例中有一个缺少想法。它应该看起来像这样：

Actually there was one missing think in example. It should look like this:

string& GetString() {
  static string strFilename;
  return strFilename;
}

extern "C" {
  void PrintToScreen() {
    printf("String: %s\n", GetString().c_str())
  }
}

奇怪。无论如何，我重构了这样的东西：

Strange. Anyway, I refactored it to something like this:

extern "C" {
  string* GetString() {
    static string strFilename;
    return &strFilename;
  }

  void PrintToScreen() {
    printf("String: %s\n", GetString()->c_str())
  }
}

现在它没有问题。

对我来说，编译器并不抱怨，似乎很奇怪。

感谢他们的贡献，问题现在解决了。

And it works without problem now.
Still it seems strange to me that compiler was not complaining.
Thanks to all for their contribution, problem is solved now.

--------------- -------------------编辑------------------------------

---------------------------------- EDIT ----------------------------------

我以后再次遇到这个问题，所以它不是正确的修正。\\
真正的问题是一些singleton被初始化

在类构造函数中：

I experienced this problem again later so it was not proper fix.
The real problem was some singleton which was initialized
in meantime and had in the class constructor:

GetString() = "";

所以，简单的问题，但很难跟踪...

So, simple problem, but really hard to track...

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