Intermediate C++ Game Programming Tutorial 25

Function pointers allow you to store and change what function is to be called dynamically at runtime. Combine that with containers to have collections of actions/behaviors that can be selected from, and then jam in the power std::function to allow you to wrap up pretty much any kind of callable thing and bring them together in one container. Groovy.

Topics Covered

• Function pointers
• Functionoids
• std::function

Bonus Video

• Pointers to member functions
• std::bind
• std::mem_fn
• std::reference_wrapper (a little)
• Pointer to static member function (it's the same as for normal funcy bois)
• You can use normal func pointers for lambdas if they do not capture anything

Video Timestamp Index

Tutorial 25

• How to switch on a string? (execute some code based on the string passed to the switch) 0:15

• One way to do this is with a map to polymorphic functors
• The Functionoid Pattern (objects that encapsulate function pointers) looks like this:

#include <iostream>
#include <memory>
#include <unordered_map>
#include <string>

struct CaseFunction
{
    virtual void operator()() const = 0;
};
struct SixtyNine : public CaseFunction
{
    void operator()() const override;
    {
         std::cout << "The S number\n";
    }
};

int main()
{
    std::unordered_map<std::string,std::unique_ptr<CaseFunction>> sw;
    sw["sixty-nine"] = std::make_unique<SixtyNine>();
    (*sw["sixty-nine"])();
    return 0;
}

• But Chili is not a fan of this:

- awkward syntax to invoke the switch,

- need to revert to unique pointer protection

- forced to put data on the heap

- need to code derived classes for new functors

• Alternative route: use function pointers 5:13
• [Work in Progress]

Tutorial 25 - Bonus

Tutorial 25 - Solution

Homework Assignment

This homework might wreck your butthole a little, but hopefully not too badly. Anyways, you get the code from the repo, you try and get it to build. The project requires Box2D, but the repo doesn't have it included so you're gonna need to wrangle it yourself. The easiest way to accomplish this is to pull in dependencies into your system with vcpkg. Some of you are probably going to run into some speed bumps at this point getting vcpkg to work on your system, but I recommend powering through and not giving the fuck up, because vcpkg is immensely useful for adding amazing libraries to your projects easily. If you search YouTube, you'll find a video of me showing how to install vcpkg and use it to grab SFML, so that might be a good starting point.

After you get that shit working, the party has just started. Then you need to figure out how the codebase actually works. Use all the tools at your disposal (debugger, Google, Box2D documentation, etc.), and don't get bogged down in unimportant distractions (you don't need to know how the triangle rendering pipeline works to understand the general simulation system, for example).

The actual tasks are, as laid out in the video:

1. Implement destruction of boxes when two boxes with the same color trait hit each other
2. Implement a box splitting mechanic
3. Implement a pattern matching collision event manager based on std::unordered_map (this is the main task that ties into Intermediate 25)
4. Use the pattern matching system together with box splitting and other effects to define simulation with various action that occur based on what colors of boxes collide with each other.

Chili will hook you up with a solution video that A) explains the starting codebase in some detail and B) goes over the process of implementing all the the above.

The repo: GitHub

See also

• Intermediate C++ Game Programming Series