Probably Dance

C++11 made the {} syntax for initializing aggregates more widely usable. You can now also use it to call constructors and to initialize with a std::initializer_list. It also allows you to drop the type name in some circumstances. The general recommendation seems to be that you use it as much as possible. But when I have started doing that I have found that it sometimes doesn’t do what I want, and that it may make maintenance more difficult.

Here’s what it looks like:

#include <iostream>

struct Widget
{
    Widget(int m) : m{m} {}
    operator int() const { return m; }
    int m;
};

Widget decrement_widget(Widget w)
{
    return { w - 1 };
}

int main()
{
    int a{1};
    std::cout << a << std::endl; // "1"
    Widget w{5};
    std::cout << w << std::endl; // "5"
    std::cout << decrement_widget({5}) << std::endl; // "4"
}

It can be used to initialize everything (hence the name uniform initialization) and as you can see it makes some code more convenient because I don’t even need the name any more if the compiler should know it.

It makes your code look a bit weird at first because you have squiggly braces everywhere, but after a while I found that I prefer it because it sets initialization apart from function calls.

But I have stopped using it and I recommend that you don’t use it either.

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As I wrote in my last post, I consider std::function to be a very important class that will change how you design your code, because it means that you have to use inheritance less often. In that post I was very impressed with the performance of std::function when compiled with optimizations. Unfortunately std::function can be far slower than a virtual function call in debug.

I wanted a std::function implementation that doesn’t have too big of a performance impact on your application when debugging it, so I wrote my own. It is also faster than all other implementations that I could find in release mode.

The code is in the public domain (I want all library writers to start using it) and here is a download link.

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In C++11 there is a new class in the standard library called std::function. It allows you to store anything that is callable. For example

#include <functional>
#include <iostream>
void print_world()
{
    std::cout << "World!" << std::endl;
}

int main()
{
    std::function<void ()> hello_world = []{ std::cout << "Hello "; };
    hello_world();
    hello_world = &print_world;
    hello_world();
}

Prints “Hello World!” as you would expect. The assignment to and storage of the two different types is handled internally by the std::function.

The amazing thing is that calling a std::function is fairly cheap: It’s one virtual function call. That has wide implications for how you will use (or not use) virtual functions in the future. In many cases where you used to have to use a virtual function it is now better to use a std::function. For example for an update loop.

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