How to capture smart pointer in lambda?

Question

What is the best way to capture smart pointer in lambda? One of my attempts led to an error after use.

Code example:

#include <cstring> #include <functional> #include <memory> #include <iostream> std::function<const char *(const char *)> test(const char *input); int main() { std::cout.sync_with_stdio(false); std::function<const char *(const char *)> a = test("I love you"); const char *c; while ((c = a(" "))){ std::cout << c << std::endl; } return 0; } std::function<const char *(const char *)> test(const char *input) { char* stored = strdup(input); char *tmpstorage = nullptr; std::shared_ptr<char> pointer = std::shared_ptr<char>(stored, free); return [=](const char * delim) mutable -> const char * { const char *b = strtok_r(stored, delim, &tmpstorage); stored = nullptr; return b; }; } 

not working as shown in AddressSanitizer.

Answer 1

A lambda (even one with a universal capture like [=] ) actually only captures the variables used in its definition. Since pointer never used inside a lambda in your example, it is not written, and therefore, when it goes out of scope, it is called by the last common pointer, referencing stored and free() .

If you want to capture pointer , you can force it to use:

 return [=](const char * delim) mutable -> const char * { pointer; const char *b = strtok_r(stored, delim, &tmpstorage); stored = nullptr; return b; }; 

However, these are pretty hacks. You want your functor to be efficient and with non-trivial government. For me, this is a strong indicator, and the actual named class (and not lambda) will be fine. So I would change it like this:

 std::function<const char *(const char *)> test(const char *input) { struct Tokenizer { std::shared_ptr<char> pointer; char* stored; char* tmpstorage; explicit Tokenizer(char* stored) : pointer(stored, free), stored(stored), tmpstorage(nullptr) {} const char* operator() (const char * delim) { const char *b = strtok_r(stored, delim, &tmpstorage); stored = nullptr; return b; } }; return Tokenizer(strdup(input)); } 

Answer 2

Just write down the variable by value and let the copy constructor and destructor worry about the semantics of ownership - about which smart pointers.