c++ - Avoid memory allocation with std::function and member function -


this code illustrating question.

#include <functional> struct mycallback {     void fire() {     } };  int main() {     mycallback cb;     std::function<void(void)> func = std::bind(&mycallback::fire, &cb); }

experiments valgrind shows line assigning func dynamically allocates 24 bytes gcc 7.1.1 on linux.

in real code, have few handfuls of different structs void(void) member function gets stored in ~10 million std::function<void(void)>.

is there way can avoid memory being dynamically allocated when doing std::function<void(void)> func = std::bind(&mycallback::fire, &cb); ? (or otherwise assigning these member function std::function)

unfortunately, allocators std::function has been dropped in c++17.

now accepted solution avoid dynamic allocations inside std::function use lambdas instead of std::bind. work, @ least in gcc - has enough static space store lambda in case, not enough space store binder object. 

std::function<void()> func = [&cb]{ cb.fire(); };     // sizeof lambda sizeof(mycallback*), small enough

as general rule, implementations, , lambda captures single pointer (or reference), avoid dynamic allocations inside std::function technique (it better approach other answer suggests).

keep in mind, work need guarantee lambda outlive std::function. obviously, not possible, , sometime have capture state (large) copy. if happens, there no way eliminate dynamic allocations in functions, other tinker stl (obviously, not recommended in general case, done in specific cases).


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