In C# or Java, the following does not compile, because I "forgot" the where part in class declaration, that specifies that T is instance of something that defines the add method.
class C<T> {
T make(T t) {
return t.add(t);
}
}
I'd like to get similar compile-time check in C++, if I specify incomplete requires for a template argument.
template <typename T>
requires true
class C {
public:
T make() {
return T{};
}
};
I'd like to get a compile error for the C++ code above, stating that method c.make relies on T being default-constructible, which is however not captured in the requires constraints on T.
Can I get the compiler to check that my set of requires constraints is sufficient to cover everything the class implementation does?
I am using gcc (GCC) 10.0.0 20191207 (experimental) for this.
What I want is called definition checking, and apparently it is not possible currently
C++2a concepts (formerly known as “Concepts Lite” and/or the Concepts TS) famously do not support “definition checking.” The idea of definition checking is that the programmer might write [...]
https://quuxplusone.github.io/blog/2019/07/22/definition-checking-with-if-constexpr/
8.2 Definition checking
Concepts currently do not prevent a template from using operations that are not specified in the requirements. Consider:
template<Number N>
void algo(vector<N>& v){
for (auto& x : v) x%=2;
}
Our Number concept does not require %=, so whether a call of algo succeeds will depend not just on what is checked by the concept, but on the actual properties of the argument type: does the argument type have %=? If not, we get a late (instantiation time) error.
Some consider this a serious error. I don’t [...]
http://www.w.stroustrup.com/good_concepts.pdf
The current proposal checks interfaces and that's where the main benefits for users are, but not template definitions. That has been explicit from the start.
https://isocpp.org/blog/2016/02/a-bit-of-background-for-concepts-and-cpp17-bjarne-stroustrup