What happens if you inline a function that calls it self in C++

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First I thought the compile time would take forever, or I take a weird error, but that didn't happen. The code runs for a while and then crashes.

This is my code:

#include <iostream>

inline void say_hello()
{
    std::cout << "hello\n";
    say_hello();
}

int main()
{
    say_hello();
}

I thought the compiler will convert it to something like this:

#include <iostream>

int main()
{
    std::cout << "hello\n";
    std::cout << "hello\n";
    std::cout << "hello\n";
    std::cout << "hello\n";
    std::cout << "hello\n";
    std::cout << "hello\n";
    std::cout << "hello\n";
    std::cout << "hello\n";
    std::cout << "hello\n";
    std::cout << "hello\n";
    std::cout << "hello\n";
    std::cout << "hello\n";
    std::cout << "hello\n";
    std::cout << "hello\n";
    std::cout << "hello\n";
    // and crash because my storage is filled
}

But, I think the compiler ignored the inline keyword.

visual-c++ c++
1 Answers

In modern C++, the inline specifier is only a suggestion to the compiler that you might want to inline the function. The compiler is not required to comply.

For your specific compiler, please see Visual Studio Inline Functions (C++). You seem to "want" the __forceinline decorator combined with #pragma inline_recursion(on). This will inline to a depth of 16, but that is modifiable as well. I hope it's obvious why this is all a bad idea. Note that this is ALL compiler specific and does not apply to gcc.

__forceinline can fail for a variety of reasons, but the ones that might apply to you:

  • the function is recursive (what you have) and #pragma inline_recursion(on) is not set
  • the program is compiled with /Ob0 (the default for debug builds, which you might have set)

If you want to recurse to a level different than 16 (the default), you can use the inline_depth pragma to specify.

Your function would end up looking like this (untested):

#include <iostream>

#pragma inline_depth(9000)
#pragma inline_recursion(on)
__forceinline void say_hello()
{
    std::cout << "hello\n";
    say_hello();
}

int main()
{
    say_hello();
}
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