Working with lambdas is great in c++, but sometimes you want to make recursive lambdas, there are a few pitfalls that I've been in. I'm listing them here to pull me (or you!) out in the future :)

Gotcha's

bad_function_call issue

The following code example produces a bad function call due to capturing the lambda by value:

// Declaration
std::function<void(const SBone*, const core::vector3df&, const core::quaternion&)> f_build;

f_build = [f_build](const SBone* bone, const core::vector3df& pos, const core::quaternion& rot)
{
    // This will cause a "bad call" due to incomplete capture of f_build
    f_build(bone, pos, rot);
};

Corrected Code

The issue is resolved by capturing the lambda by reference:

// Declaration
std::function<void(const SBone*, const core::vector3df&, const core::quaternion&)> f_build;

f_build = [&f_build](const SBone* bone, const core::vector3df& pos, const core::quaternion& rot)
{
    // This will correctly call the fully initialized f_build
    f_build(bone, pos, rot);
};

Explanation

Here's why capturing by value causes a "bad call":

• Copy of f_build: When capturing by value, the lambda captures a copy of the f_build variable. However, at the point of the lambda's creation, f_build is not fully initialized. It is still being defined, leading to an incomplete or invalid function.
• Uninitialized or Incomplete State: The lambda inside the f_build variable refers to an incomplete or invalid function, which results in a "bad call" when the lambda tries to invoke itself.
• Reference to f_build: Capturing f_build by reference ([&f_build]) ensures that the lambda uses the actual f_build variable, which will eventually hold the fully initialized lambda function.
• Correct Recursive Call: By capturing by reference, the recursive call inside the lambda correctly invokes the complete and fully initialized lambda function, allowing recursion to work as intended.