[C++ compile time assertions]: Can we throw a compilation error if some condition is not met?

问题

I wrote a function:

template<int N> void tryHarder() {
    for(int i = 0; i < N; i++) {
        tryOnce();
    }
}

but I only want it to compile if N is in between 0 and 10. Can I do it? How?

回答1:

You can do it with static_assert declaration:

template<int N> void tryHarder() {

    static_assert(N >= 0 && N <= 10, "N out of bounds!");

    for(int i = 0; i < N; i++) {
        tryOnce();
    }
}

This feature is only avaliable since C++11. If you're stuck with C++03, take a look at Boost's static assert macro.

The whole idea of this are nice error messages. If you don't care for those, or can't even affor boost, you could do something as follows:

template<bool B>
struct assert_impl {
    static const int value = 1;
};

template<>
struct assert_impl<false> {
    static const int value = -1;
};

template<bool B>
struct assert {
    // this will attempt to declare an array of negative
    // size if template parameter evaluates to false
    static char arr[assert_impl<B>::value]; 
};

template<int N>
void tryHarder()
{
    assert< N <= 10 >();
}

int main()
{
    tryHarder<5>();  // fine
    tryHarder<15>();  // error, size of array is negative
}

回答2:

For pre C++11 compilers, you could implement a template parameter constraint on the non-type parameter N.

For a description of how to do this, please see http://stroustrup.com/bs_faq2.html#constraints

回答3:

Combined with the answers given so far already, the lower bound can be covered, too, by using unsigned int as template type. Negative values, if applied, will be converted to unsigned values high enough that they will be covered with the static_assert or the pre-C++11 solution anyway.

unsigned int additionally gives already semantically a hint that negative values shall not be applied to this template, which is why it (possibly) should be preferred in the specific case...

回答4:

#if !defined(__cplusplus)
#error C++ compiler required.
#endif

This is just an example.

Here is the source link: http://msdn.microsoft.com/en-us/library/c8tk0xsk(v=vs.71).aspx

All i am saying is that you can use #error also

It is a directive

Edit @Pratik Chowdhruy: I agree with Paul R. This does not answer the question directly. Sorry to the community

来源：https://stackoverflow.com/questions/16192575/c-compile-time-assertions-can-we-throw-a-compilation-error-if-some-conditio

标签

c++

templates

assert

compile-time