Probably Dance

I can program and like games

Month: February, 2013

February 20, 2013

Handmade Coroutines for Windows

In a previous post I implemented coroutines using the ucontext.h header. In this post I will show a replacement that should be much faster. I will also provide a Windows implementation.

I like to start off with some code, so here is the complete code for switching the stack in Linux:

switch_to_context:
    pushq %rbp
    movq %rsp, %rbp
    // store rbx and r12 to r15 on the stack. these will be restored
    // after we switch back
    pushq %rbx
    pushq %r12
    pushq %r13
    pushq %r14
    pushq %r15
    movq %rsp, (%rdi) // store stack pointer
switch_point:
    // set up the other guy's stack pointer
    movq %rsi, %rsp
    // and we are now in the other context
    // restore registers
    popq %r15
    popq %r14
    popq %r13
    popq %r12
    popq %rbx
    popq %rbp
    retq

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February 2, 2013

The problems with uniform initialization

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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