template errors

Question

Hi,

I heard C++ templates wont generate errors until they are used. Is it true ? Can someone explain me how they work ?

Answer 1

From here,

From the point of view of the compiler, templates are not normal functions or classes. They are compiled on demand, meaning that the code of a template function is not compiled until an instantiation with specific template arguments is required. At that moment, when an instantiation is required, the compiler generates a function specifically for those arguments from the template.

Hope it helps..

Answer 2

They generate compiler errors when they are compiled. They are compiled separately for each actual parameter passed as the template argument(s) (this is unlike Java Generics), e.g., if I have:

template <typename T> class foo { ... }

and

int main() {
  foo<char> c;
  foo<int> i ;
}

the template foo gets compiled twice, once for chars, once for ints.

If you never (directly or indirectly) instantiated or used template foo, it wouldn't be compiled and you'd not see any compiler errors.

Once compiled, they're just "normal" C++ code, and like any code, can generate runtime errors.

Answer 3

Templates follow two phase compilation model.

struct X{
private:
   void f(){}
};

template<class T> void f(T t){
   int;   // gives error in phase 1 (even if f(x) call is commented in main)
   t.f(); // gives error only when instantiated with T = X, as x.f() is private, in phase 2
}

int main(){
   X x;
   f(x);
}

Answer 4

Conceptually, at the highest level

template <Type value, class Y, ...>
...fn-or-class...

may be usefully compared to

#define FN_OR_CLASS(VALUE, TYPE_Y, ...) \
...fn-or-class...

Both basically wait until called/instantiated then substitute the specified types and values to produce tailored code with full compile-time optimisation for those values. But, templates differ from #defines in that they're proper compile-stage constructs that can be enclosed in namespaces, must satisfy the lexer, and not all of a class template is generated when the first instantiation is seen - rather, functions are generated on an as-needed basis.

When the compiler first encounters a template, it does a rough check that the template's content could make sense for some hypothetical instantiation. Later, when it encounters a specific instantiation, then for class templates only the functions that are used are further checked to make sure they can be compiled with the specific parameters in use. This does mean that a class template can appear - for some limited usage - to support instantiation with specific parameters, but if you start using some other functions in the template API then suddenly you can find that it can't be compiled with that presumed-suitable parameter... can force you to redesign your usage rather late in the day. That is one of the reasons that C++0x had planned to introduce Concepts: they elegantly allow templates to check that parameters meet all the template's expectations - if they allow any instantiation, then the user can assume that the full API of the template can be used.

template <class T>
struct X
{
    void f() { }
    void g() { T::whatever(); } // only error if g() called w/o T::whatever
};

int main()
{
    X<int> x;
    x.f();
    // x.g(); // would cause an error as int::whatever() doesn't exist...
}

The SFINAE (substitution failure is not an error) technique can then allow the compiler to select between multiple nearly-matching functions based on the actual instantiating template parameters. This can be used to implement basic compile-time introspection, such as "does this class have a member function fn(int)?".