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c++c++11templatesstatic-assert

Use static_assert to ensure template parameter is only used *at most* once

Is it possible to ensure a certain template parameter (or combination of template parameters) is used - at most - once? Would it be possible to track the number of times it's used, and throw a static_assert error if it was used more than once?

As an example - let's say I wanted to create a static, template-based GPIO class:

template<uint8_t Port, uint8_t Pin> class Gpio {};

One could feasibly lay out their entire board in a single Board class:

struct Board {
  Gpio<1, 1> myInputPin;
  Gpio<1, 2> myOutputPin;

  Gpio<2, 0> ledR;
  Gpio<2, 1> ledG;
  Gpio<2, 2> ledB;
};

For any given port/pin combination, it only makes sense for a single GPIO template to be instantiated for that port and pin. If someone creates two different GPIO instances for the same port/pin combination, it likely represents a conflict, and I would like to catch this error during compilation.

Is this possible?

Solution

  • You can solve this problem with a macro:

    #define GPIO(Port, Pin) \
    friend void _gpio_ ## Port ## _ ## Pin(){} \
    Gpio<Port, Pin>

    Then if you use GPIO(2, 2) twice, the compiler will say something like this:

    error: redefinition of '_gpio_2_2'
  GPIO(2, 2) ledX;
  ^

<source>:14:3: note: previous definition is here
  GPIO(2, 2) ledB;
  ^

    Demo: https://godbolt.org/z/ronV0u