Do you really need a main() in C++?_问答_开发者

From what I can tell you can kick off all the action in a constructor when you create a global object. So do you really need a main() function in C++ or is it just legacy?

I can understand that it could be considered bad practice to do so. I'm just as开发者_Python百科king out of curiosity.

If you want to run your program on a hosted C++ implementation, you need a main function. That's just how things are defined. You can leave it empty if you want of course. On the technical side of things, the linker wants to resolve the main symbol that's used in the runtime library (which has no clue of your special intentions to omit it - it just still emits a call to it). If the Standard specified that main is optional, then of course implementations could come up with solutions, but that would need to happen in a parallel universe.

If you go with the "Execution starts in the constructor of my global object", beware that you set yourself up to many problems related to the order of constructions of namespace scope objects defined in different translation units (So what is the entry point? The answer is: You will have multiple entry points, and what entry point is executed first is unspecified!). In C++03 you aren't even guaranteed that cout is properly constructed (in C++0x you have a guarantee that it is, before any code tries to use it, as long as there is a preceeding include of <iostream>).

You don't have those problems and don't need to work around them (wich can be very tricky) if you properly start executing things in ::main.

As mentioned in the comments, there are however several systems that hide main from the user by having him tell the name of a class which is instantiated within main. This works similar to the following example

class MyApp {
public:
  MyApp(std::vector<std::string> const& argv);

  int run() {
      /* code comes here */
      return 0;
  };
};

IMPLEMENT_APP(MyApp);

To the user of this system, it's completely hidden that there is a main function, but that macro would actually define such a main function as follows

#define IMPLEMENT_APP(AppClass) \
  int main(int argc, char **argv) { \
    AppClass m(std::vector<std::string>(argv, argv + argc)); \
    return m.run(); \
  }

This doesn't have the problem of unspecified order of construction mentioned above. The benefit of them is that they work with different forms of higher level entry points. For example, Windows GUI programs start up in a WinMain function - IMPLEMENT_APP could then define such a function instead on that platform.

Yes! You can do away with main.

Disclaimer: You asked if it were possible, not if it should be done. This is a totally un-supported, bad idea. I've done this myself, for reasons that I won't get into, but I am not recommending it. My purpose wasn't getting rid of main, but it can do that as well.

The basic steps are as follows:

Find crt0.c in your compiler's CRT source directory.
Add crt0.c to your project (a copy, not the original).
Find and remove the call to main from crt0.c.

Getting it to compile and link can be difficult; How difficult depends on which compiler and which compiler version.

Added

I just did it with Visual Studio 2008, so here are the exact steps you have to take to get it to work with that compiler.

Create a new C++ Win32 Console Application (click next and check Empty Project).
Add new item.. C++ File, but name it crt0.c (not .cpp).
Copy contents of C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\crt\src\crt0.c and paste into crt0.c.
Find mainret = _tmain(__argc, _targv, _tenviron); and comment it out.
Right-click on crt0.c and select Properties.
Set C/C++ -> General -> Additional Include Directories = "C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\crt\src".
Set C/C++ -> Preprocessor -> Preprocessor Definitions = _CRTBLD.
Click OK.
Right-click on the project name and select Properties.
Set C/C++ -> Code Generation -> Runtime Library = Multi-threaded Debug (/MTd) (*).
Click OK.
Add new item.. C++ File, name it whatever (app.cpp for this example).
Paste the code below into app.cpp and run it.

(*) You can't use the runtime DLL, you have to statically link to the runtime library.

#include <iostream>

class App
{
    public: App()
    {
        std::cout << "Hello, World! I have no main!" << std::endl;
    }
};

static App theApp;

Added

I removed the superflous exit call and the blurb about lifetime as I think we're all capable of understanding the consequences of removing main.

Ultra Necro

I just came across this answer and read both it and John Dibling's objections below. It was apparent that I didn't explain what the above procedure does and why that does indeed remove main from the program entirely.

John asserts that "there is always a main" in the CRT. Those words are not strictly correct, but the spirit of the statement is. Main is not a function provided by the CRT, you must add it yourself. The call to that function is in the CRT provided entry point function.

The entry point of every C/C++ program is a function in a module named 'crt0'. I'm not sure if this is a convention or part of the language specification, but every C/C++ compiler I've come across (which is a lot) uses it. This function basically does three things:

Initialize the CRT
Call main
Tear down

In the example above, the call is _tmain but that is some macro magic to allow for the various forms that 'main' can have, some of which are VS specific in this case.

What the above procedure does is it removes the module 'crt0' from the CRT and replaces it with a new one. This is why you can't use the Runtime DLL, there is already a function in that DLL with the same entry point name as the one we are adding (2). When you statically link, the CRT is a collection of .lib files, and the linker allows you to override .lib modules entirely. In this case a module with only one function.

Our new program contains the stock CRT, minus its CRT0 module, but with a CRT0 module of our own creation. In there we remove the call to main. So there is no main anywhere!

(2) You might think you could use the runtime DLL by renaming the entry point function in your crt0.c file, and changing the entry point in the linker settings. However, the compiler is unaware of the entry point change and the DLL contains an external reference to a 'main' function which you're not providing, so it would not compile.

Generally speaking, an application needs an entry point, and main is that entry point. The fact that initialization of globals might happen before main is pretty much irrelevant. If you're writing a console or GUI app you have to have a main for it to link, and it's only good practice to have that routine be responsible for the main execution of the app rather than use other features for bizarre unintended purposes.

Well, from the perspective of the C++ standard, yes, it's still required. But I suspect your question is of a different nature than that.

I think doing it the way you're thinking about would cause too many problems though.

For example, in many environments the return value from main is given as the status result from running the program as a whole. And that would be really hard to replicate from a constructor. Some bit of code could still call exit of course, but that seems like using a goto and would skip destruction of anything on the stack. You could try to fix things up by having a special exception you threw instead in order to generate an exit code other than 0.

But then you still run into the problem of the order of execution of global constructors not being defined. That means that in any particular constructor for a global object you won't be able to make any assumptions about whether or not any other global object yet exists.

You could try to solve the constructor order problem by just saying each constructor gets its own thread, and if you want to access any other global objects you have to wait on a condition variable until they say they're constructed. That's just asking for deadlocks though, and those deadlocks would be really hard to debug. You'd also have the issue of which thread exiting with the special 'return value from the program' exception would constitute the real return value of the program as a whole.

I think those two issues are killers if you want to get rid of main.

And I can't think of a language that doesn't have some basic equivalent to main. In Java, for example, there is an externally supplied class name who's main static function is called. In Python, there's the __main__ module. In perl there's the script you specify on the command line.

If you have more than one global object being constructed, there is no guarantee as to which constructor will run first.

If you are building static or dynamic library code then you don't need to define main yourself, but you will still wind up running in some program that has it.

If you are coding for windows, do not do this.

Running your app entirely from within the constructor of a global object may work just fine for quite awhile, but sooner or later you will make a call to the wrong function and end up with a program that terminates without warning.

Global object constructors run during the startup of the C runtime.
The C runtime startup code runs during the DLLMain of the C runtime DLL
During DLLMain, you are holding the DLL loader lock.
Tring to load another DLL while already holding the DLL loader lock results in a swift death for your process.

Compiling your entire app into a single executable won't save you - many Win32 calls have the potential to quietly load system DLLs.

There are implementations where global objects are not possible, or where non-trivial constructors are not possible for such objects (especially in the mobile and embedded realms).