Can I ignore the gcc warning: ‘Foo::m_bar’ should be initialized in the member initialization list [-Weffc++]

Question

struct Bar
{
    Bar() {}
};


struct Foo
{
    Foo() = default;
    Bar m_bar;
};

int main()
{
    Foo foo;
}

When using C++11 default

Answer 1

You can ignore or suppress the warning. This is a misinterpretation of one of the Effective C++ guidelines. The guideline says to prefer initialisation to assignment, but in your example, m_bar will be initialised. Your code is correct.

Source: Jonathan Wakely in GCC's bug tracker:

# Item 12: Prefer initialization to assignment in constructors.

Replaced by Item 4: "Make sure that objects are initialized before they're used", and G++ misinterprets the original item anyway and warns about any member without a mem-initializer, which is very annoying: there's no point initializing a std::string, it has a perfectly safe default constructor. My -Wmeminit patch for PR 2972 should replace the current warning for this item, as it only warns about members left uninitialized by the constructor.

(And as it's a known issue, there's no need to report it as a bug again.)

Answer 2

Is it alright to ignore this warning? Yes.

Is it a good idea to ignore this warning? Depends(*)

Should you file a bug to gcc? No(*)

(*)

• default constructor in fact initialises m_bar just fine, you can test that
• it a bit weird that g++ doesn't get that
• you selected very verbose warning setting
• warning is not about correctness of your code, rather about style
• you can't correct this and keep default constructor for Foo and custom constructor for Bar

man g++, section -Weffc++

Warn about violations of the following style guidelines from Scott Meyers’ Effective C++ book:

• Item 11: Define a copy constructor and an assignment operator for classes with dynamically allocated memory.
• Item 12: Prefer initialization to assignment in constructors.
• Item 14: Make destructors virtual in base classes.
• Item 15: Have "operator=" return a reference to *this.
• Item 23: Don’t try to return a reference when you must return an object.

Also warn about violations of the following style guidelines from Scott Meyers’ More Effective C++ book:

• Item 6: Distinguish between prefix and postfix forms of increment and decrement operators.
• Item 7: Never overload "&&", "││", or ",".

When selecting this option, be aware that the standard library headers do not obey all of these guidelines.