Boost C++ Application Development Cookbook - Second Edition

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The boost::call_once may pass parameters to the function to call:

#include <iostream>void once_printer(int i) {    static boost::once_flag flag = BOOST_ONCE_INIT;    boost::call_once(        flag,        [](int v) { std::cout << "Print once " << v << '\n'; },        i // <=== Passed to lambda from above.    );    // ...}

Now, if we call once_printer function in a loop:

int main() {    for (unsigned i = 0; i < 10; ++i) {        once_printer(i);    }}

Only a single line will be in the output:

Print once 0

C++11 has a std::call_once and std::once_flag in the <mutex> header. Unlike the Boost version, the standard library version of the once_flag does not require initialization via a macro, it has a constexpr constructor. As usual, Boost version is usable on pre-C++11 compilers, ...