c++,function,c++11,lambda,allegro , How can I simulate a nested function without lambda expressions in C++11?


How can I simulate a nested function without lambda expressions in C++11?

Question:

Tag: c++,function,c++11,lambda,allegro

I have the following code:

int main(int argc, char **argv)
{
    App app(800, 600);
    app.add_event_scene(Scene("Event Plot", event_plot));

    Image x("sample.png");
    struct foo { static void visual_plot() { x.draw(); } }; // Error.

    app.add_visual_scene(Scene("Visual Plot", foo::visual_plot));
    app.run();
    return 0;
}

And I get the following error:

||=== Build: Debug in Joy-Plus-Plus (compiler: GNU GCC Compiler) ===|
G:\Development\Game-Development\CB\Joy-Plus-Plus\main.cpp|54|error: use of local variable with automatic storage from containing function|
G:\Development\Game-Development\CB\Joy-Plus-Plus\main.cpp|53|error:   'Image x' declared here|
||=== Build failed: 2 error(s), 0 warning(s) (0 minute(s), 0 second(s)) ===|

I'm writing a multimedia/game engine for the Allegro 5 library, and I've abstracted the drawing part of the main-loop (As well as the event parts) into "scene" objects with plots (Functions). Each procedure is passed to the App, so that it gets "run" inside the main-loop. The problem is, the "C++ approach" does not work:

Image x("sample.png");

void visual_plot()
{
    x.draw(); // Problem.
}

int main(int argc, char **argv)
{
    App app(800, 600);
    app.add_event_scene(Scene("Event Plot", event_plot));
    app.add_visual_scene(Scene("Visual Plot", visual_plot));
    app.run();
    return 0;
}

Although the code runs, this happens:

enter image description here

And if I put the x inside the visual_plot, the image is loaded normally:

enter image description here

But now I have a huge performance problem, since a new Image object is being created at each main-loop (And it's not long until the whole thing freezes).

The image is not found when I put it outside the scope of the function because it must come after the initialization of the App, but since I have a typedef function pointer in Scene that takes that function as an argument, I also must give it a void function. The problem is that I can't create local / nested functions in C++ (After the initialization of the App). So, in order to avoid the problem, I've tried the obvious (Lambda expression / closure):

int main(int argc, char **argv)
{
    App app(800, 600);
    app.add_event_scene(Scene("Event Plot", event_plot));
    Image x("sample.png");
    app.add_visual_scene(Scene("Visual Plot", [&x]()->void{x.draw();}));
    app.run();
    return 0;
}

The problem is that the second argument of the constructor of Scene takes a function pointer:

typedef void(*plot)();
typedef map<string, plot> act;

class Scene
{
private:
    string name;
    plot p;
public:
    Scene(string name, plot p);
    ~Scene() {};
    string get_name();
    plot get_plot();
    void set_name(string value);
    void set_plot(plot value);
};

And since functions cannot be passed as parameters, and get decayed to pointers, the same also applies to the lambda expression (Which is not a function), so I get the following error:

G:\Development\Game-Development\CB\Joy-Plus-Plus\main.cpp|52|error: no matching function for call to 'Scene::Scene(const char [12], main(int, char**)::__lambda0)'|

Facing such a tragedy, how can I simulate a nested function in C++11? Since simulating like this answer does not work.

OBS: I agree that it could be a design problem, but I pretty much don't see it that way. For me, C++ just don't want me to pass that bloody function as a parameter by any means (So, I ask for the help of you long C++ Wizards).


Answer:

Simply put the image inside the visual_plot function and make it static:

void visual_plot()
{
    static Image img("sample.png");
    x.draw(); // Problem.
}

This will initialize img the first time visual_plot is called, and only then. This will solve both the performance problem and the "it must be initialized after app.run()" issue.


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