zivsha/cpplazy

:snail: A C#-like generic class for lazy initialization

cpplazy

A hedaer-only, C#-like, generic class for lazy initialization in C++17.

Simple API

    lazy<std::string> lazy_string{ [] { return "very expensive initialization here...."s; } };
    //Same API as `std::optional"
    std::string data = *lazy_string;
    std::string data2 = lazy_string->value();

Generic lazy initialized type

    lazy<std::array<int, 6>> lazy_fib_seq{ [] { return std::array<int, 6>{ 0, 1, 1, 2, 3, 5 }; } };
    //First 6 numbers of fibbonaci have not been created 
    std::array<int, 6> fib_seq = *lazy_fib_seq; //lazy object is initialized at (and only at) first usage
    std::array<int, 6> fib_seq2 = lazy_fib_seq->value(); //Returning the value, without re-initializing

Thread safe access

    lazy<int> the_answer_to_life_the_universeand_everything{ [] { std::cout << "Computing answer...Finished"  << std::endl; return 42; } };

    auto ask_question = [&] { std::cout << the_answer_to_life_the_universeand_everything->value() << std::endl; };
    std::thread hitchhiker_1(ask_question);
    std::thread hitchhiker_2(ask_question);

    hitchhiker_1.join();
    hitchhiker_2.join();

    //(Possible) Output:
    //Computing answer...Finished
    //42
    //42

Failed initialization handling

    using namespace std;

    cpplazy::lazy<string> lazy_config_value{ []() ->string { throw invalid_argument("can't open config file"); } };

    string the_value = lazy_config_value->value_or("oops"); //value will be fallback to: "oops"

    try
    {
        the_value = lazy_config_value->value();   //Will throw
        the_value = *lazy_config_value;           //Will throw
    }
    catch (const bad_optional_access& ex) 
    {
        std::cout << "'invalid_argument' was thrown when initializing, but"
                     " `bad_optional_access` is thrown when taking the value" << std::endl;
    }

Installation

Simply copy cpplazy.hpp to your project. See demo project and tests for examples.