在 C + + 中，main()中没有声明任何内容，但是在编译之后仍然有一个可以工作的应用程序，这是如何做到的呢？

在一次采访中，我遇到了这样一个问题:

您的朋友已经给了您一个源代码文件，该文件将打印 控制台上的斐波那契数字 是空的，里面没有任何语句。

解释这是如何实现的(提示: 全局实例!)

我真的很想知道，这样的事情怎么可能发生！

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Things can happen during initialization of global/static variables. The code will be trigger at the application start.

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Yes it is possible. You need to declare a global instance of an object that calculates the Fibonacci numbers in the object constructor.

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All[*] constructors for file-scope objects get called before reaching main, as do all initializer expressions for non-object file-scope variables.

Edit: Also, all[*] destructors for all file-scope objects get called in reverse order of construction after main exits. You could, theoretically, put your fibonacci program in an object's destructor.

[*] Note that 'all' ignores the behavior of dynamically loading and unloading libraries that your program wasn't directly linked with. Those technically are outside the base C++ language, though.

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Hope this helps

class cls
{
public:
cls()
{
// Your code for fibonacci series
}
} objCls;


int main()
{
}

So as soon as a global variable of the class is declared, the constructor is called and there you add the logic to print out the Fibonacci series.

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最佳答案

It is most likely implemented as (or a variant of it):

 void print_fibs()
{
//implementation
}


int ignore = (print_fibs(), 0);


int main() {}

In this code, the global variable ignore has to be initialized before entering into main() function. Now in order to initialize the global, print_fibs() needs to be executed where you can do anything — in this case, compute fibonacci numbers and print them! A similar thing I've shown in the following question (which I had asked long back):

Note that such code is not safe and should be best avoided in general. For example, the std::cout object may not be initialized when print_fibs() is executed, if so then what would std::cout do in the function? However, if in other circumstances, it doesn't depend on such initialization order, then it is safe to call initialization functions (which is a common practice in C and C++).

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I know some examples like that you tell. One way to get it is using the template metaprogramming. Using it you can move some compute process to the compilation.

Here you can get an example with the Fibonacci numbers

If you use it in a static class constructor and you can write the numbers without need to write any code in the main function.

Hope it helps you.


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