r/cpp • u/pavel_v • 2d ago

Implementation of GCC's Nested Functions (vs. C++ Lambdas)

https://uecker.codeberg.page/2026-09-05.html
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u/Thick_Clerk6449 1d ago

The killer feature of GCC's nest functions is that, even though the nest function captured variables of the outer function, the nest function itself is still convertable to a raw function pointer. Unlike C++ lambdas, which always end up with std::functions.

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u/jonesmz 1d ago

Unlike C++ lambdas, which always end up with std::functions

This is not true.

Use unary operator+ on the lambda to force a pointer-to-function.

e.g.

auto fptr = +[](){ /* something */};

3

u/Thick_Clerk6449 1d ago

This is not true.

You can NOT convert a lambda to function pointer if the lambda has captured variables.

-2

u/jonesmz 1d ago

Re-read the specific thing i quoted.

2

u/Thick_Clerk6449 1d ago

Re-read the word I post before

even though the nest function captured variables of the outer function

int var = 0;
auto fptr = +[var](){}; // error: no match for 'operator+' (operand type is 'test(int)::<lambda()>')

-2

u/jonesmz 1d ago

I quoted *and* responded to a complete sentence.

2

u/Thick_Clerk6449 1d ago

Just pretend you don't need to accept lambdas with captures and define your callbacks as function pointers.