C++ - [Concurrency 10] Mutable Variables

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#include <iostream>
#include <string>

class Foo {
public:
    Foo(std::string name) : name_(std::move(name)) {}

    // This function is logically "const" (it doesn't change the observable state
    // of the object), but we still want to keep a call counter.
    void do_something() const {
        // allowed because cnt_frame_ is mutable
        ++cnt_frame_;

        std::cout << "Foo(" << name_ << ") do_something called "
                  << cnt_frame_ << " times\n";
    }

    std::size_t call_count() const {
        return cnt_frame_;
    }

private:
    std::string name_;
    mutable std::size_t cnt_frame_ = 0;  // can be modified in const functions
};