Ordinary int main(){} does not write return 0; what will happen?

The conclusion may be known by looking at the above picture, without adding return 0; the compiler will automatically add one.How to prove it?

You can view the corresponding assembly code, and it is very convenient to use the godbolt.org website to view the assembly code.

As shown above, enter the C++ code, and the assembly code compiled by the compiler will be displayed in the right half. By looking at the assembly instructions behind the high-level language, we can better analyze the performance of the code.The more powerful feature of the website is that it supports almost all compilers on the market, and there are various versions.

Use this website to analyze a wave below!

Look at a screenshot with return 0:

Look at the code circled by the red box on my right. You can understand that eax is the return value of the main() function. Here you can see that the return value of the main() function is 0.

Look at the corresponding screenshot without return 0:

It can be found that the compiler still sets the value of eax to 0, which can be understood as, even if return 0 is not written in the main function, the compiler will still add a return 0 by default.

Will other non-main functions also add a return 0 by default?

Let's take a look at an ordinary function with an int return value:

Note that the eax register in func() is assigned a value of 9, that is, the return value of func() is 9.

What if return a is not added?

It can be seen that if the return statement is not written in func(), the compiler will not update the value of the eax register, and the return value of the function will not be what we expect.

Conclusion: Normally a function with a return value, the return value will exist somewhere, it may be on the stack or in the register. If you don't give it a return value, the return value obtained externally may beA strange value, because we don't know what data that address is.

But, except for the main function, because if the main function does not return, the compiler will add a return 0 by default; but I personally think this is indeed a bad habit, the mian function will make this mistake, and other functions are estimated to beThat's it, so it's better to add return xxx to all functions with return values.