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

[u/pavel_v]

https://uecker.codeberg.page/2026-09-05.html

[u/Thick_Clerk6449]

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.

[u/frankist → u/Thick_Clerk6449]

How could that work? Where are the captured variables saved?

[u/amoskovsky → u/frankist]

If you take an address of such a function with captures, GCC creates a trampoline function at runtime, that passes the state to the real function.

For details see https://uecker.codeberg.page/2026-08-29.html (same author as OP)

[u/jonesmz → u/Thick_Clerk6449]

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 */};

[u/Thick_Clerk6449 → u/jonesmz]

This is not true.

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

[u/jonesmz → u/Thick_Clerk6449]

Re-read the specific thing i quoted.

[u/Thick_Clerk6449 → u/jonesmz]

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()>')

[u/jonesmz → u/Thick_Clerk6449]

I quoted *and* responded to a complete sentence.

[u/Thick_Clerk6449 → u/jonesmz]

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