CODEWITHSUNDEEP
templatestemplate-meta-programmingc++03
Zak
Zak

Reputation: 12653

compile time bounding of template parameters in c++0x

Here is a grossly over-simplified class:

template <unsigned x>
class myArray {
    int getIndex(unsigned i) { return y[i]; }
    void setIndex(unsigned i, int v) { y[i] = v; }
  private:
    int y[x];
};

How would I use template meta-programming to ensure x is less than 64 (or any arbitrary value)?

I could add the constructor:

myArray () { if (x >= 64) { throw; } }

but that is horrible...

Is there a more elegant way to bound the template parameter, so that it can be checked at compile time, instead of runtime?

NOTE: If this is possible in c++03, then I need the syntax in that form (in case it were to differ from c++11).

UPDATE

I arrived at a solution as a derivate of the examples provided by the following SO post...

C compiler asserts - how to implement?

Upvotes: 0

Views: 37

Answers (1)

Julius
Julius

Reputation: 1861

C++11 and above

In C++11 and above there is the static_assert declaration for this purpose. Your compiler then checks the condition at compile time. You can use it at class scope in the form

template <unsigned x>
class myArray {
    static_assert(x > 0 && x <= 64, "array size in (0, 64]");
};

Before C++11

In pre-C++11 there are workarounds to provide the same functionality as C++11's static_assert. Most of those workarounds provide a macro based on the following idea: if your check (compile-time expression of type bool) yields false then the macro expands to some illegal code that produces a compiliation error. Otherwise, the macro expands to an empty string. Advanced implementations allow one to specify an additional error message. If you want an out-of-the-box solution then you may have a look at BOOST_STATIC_ASSERT.

Upvotes: 2

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