C++ implicit cast - How to force a warning

Pierre Baret 0
Viewed 51

I am following a C++ training and I found out a behavior I find weird in C++ when learning about explicit keyword.

About the following snippet, it will compile and execute without any error or warning (compile with G++).

When calling Foo(5), it will automatically do an implicit conversion and actually call Foo(A(5)).

I know I can forbid this behavior by making the constructor of A explicit : explicit A(int a); but my question is :

  • Is there a way to make G++ warn about implicit conversion like that if I forgot to do so?

I tried g++ with -Wall -Wextra, I saw stuff on -Wconversion on SO but still build without any warnings.

Looking on the internet I found out some warnings seem to exist for C and ObjC, but not for C++...

enter image description here

Source: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html

The snippet:

#include <iostream>

class A{
    public:
        A(int a) 
        {
            m_a = a;
        };

        int m_a;
};

void Foo(A a)
{
    std::cout << "Hello : " << a.m_a << std::endl;
}

int main(int argc, char** argv)
{
    Foo(5);  // No Warning ?
    
    return 0;
}

Output

Hello : 5

implicit-conversion g++ c++
1 Answers

Is there a way to make G++ warn about implicit casts like that if I forgot to do so?

No, the fact that you have a choosen to have a non-explicit conversion constructor says that you want implicit conversion from int to Foo allowed.

If that is not desirable, then you always have the option of making this converting ctor explicit to forbid this completely. No way to have it both ways.

Now I realize that for code protection I should make all my CTORs explicit to avoid this, which I find a bit cumberstone

You pay for what you use.

Jason Liam0
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