Wolne std::async

std::vector < std::future < immutable_image >> imgs;
for( auto i = 0u; i < 150; ++i ) {
    imgs.emplace_back( image.set_region( std::launch::async, { 300, 250 }, { 800, 2000 }, sf::Color { 0, 255, 255 } ) );
}
std::cout << "Done!" << std::endl;

immutable_image set_region( const sf::Vector2u topleft, const sf::Vector2u bottomright, const sf::Color c ) const {
    auto transient = m_data.transient();
   
    for( auto x = topleft.x; x < bottomright.x; ++x ) {
        for( auto y = topleft.y; y < bottomright.y; ++y ) {
            transient.set( x + y * size().x, c );
        }
    }
   
    return immutable_image {
        std::move( transient.persistent() ),
        size()
    };
}
std::future < immutable_image > set_region( const std::launch policy, const sf::Vector2u topleft, const sf::Vector2u bottomright, const sf::Color color ) const {
    return std::async(
    policy,
    [ this ]( const auto tl, const auto br, const auto c ) {
        return this->set_region( tl, br, c );
    },
    topleft,
    bottomright,
    color
    );
}