Ok I see! Great point.

If I understand well, the problem happens when the method has the same name in the base and derived class. Is it a normal use case for the CRTP? I need to think about it. In general it is advised to avoid identical names for non-virtual functions across a class hierarchy (see Effective C++ item 36), but does that guideline hold for the CRTP?

When I run the code you provided with the #error I got an compile error even when the function was defined. Maybe I got your snippet wrong because the comment was formatted. Would you have a snippet in a online compiler such as coliru to make sure I get this right? Also, did you consider using a static_assert?

Hello Jonathan! Many thanks for your reply. Please find a snippet below leading to segfault:

#include
using namespace std;

template class Base {
public:
// Build interface – compile time
void test() {
D& derv = static_cast(*this);
derv.test();
}
};

class Derived : public Base {
public:
//Implement test; // We forgot to add the function
/*void test() { cout<<"Conc test"<<endl;}*/
};

// Implement client
template void client(Base a) {
a.test();
}

int main()
{
Derived d;
client(d);
return 0;
}

Playing around with a compile time assertion though could work i.e. modifying Base along the following lines throws an error (generally the logic could be wrapped inside a template for each function):

template class Base {
public:

/* Safety when using CRTP – enforce method implementation in compile time */
using F = void (D::*)();
F d1 = &D::test;

//Nullptr Check
#if (d1==0)
#error “test function not defined on Derived class”
#endif

// Build interface – compile time
void test() {
D& derv = static_cast(*this);
derv.test();
}
};

Many thanks ! Stavros

Thank you!

This looks like the most vexing parse! 🙂
On the line with:

Derived k();

This is parsed as a function declaration. Try to remove the () or replace them with {} or with auto to stick maybe, and please let me know if there are any other compile errors left. Or alternatively send over a coliru link that I could look at.

Great article! Thanks a lot for posting it. It took me a while to understand why you would need it but it finally clicked.

I was trying to incorporate this pattern to my pet project but I can’t make it work. I can’t find what’s wrong with it.

This is the error I get:
||=== Build: Debug in test (compiler: GNU GCC Compiler) ===|
In function ‘int main()’:
error: no matching function for call to ‘dosomething(Derived (&)())’
note: candidate: template void dosomething(const Base&)
note: template argument deduction/substitution failed:
note: mismatched types ‘const Base’ and ‘Derived()’

This is the code:
“`
template
struct Base {
int x;
Base():x(4){}
};

struct Derived: Base {
Derived(){}
};

template
void dosomething(Base const& b) {
std::cout << b.x << std::endl;
}

int main() {
Derived k();
dosomething(k);
}
“`

Why do you think I am getting this error?

Thanks for the nice comment. Would you have a code snippet that compiles and doesn’t work at runtime to illustrate the issue?

Thank you very much for such a clear explanation of the pattern.

I have been playing around with it and I believe I have found a serious issue in terms of type-checking.
The problem is that the interface does not force the inherited classes to provide the functions. (i.e. like it would in the case of dynamic polymorphism via pure virtual functions).
As a result if we forget to include the getValue function in a derived class (i.e. Constant42) then the code will compile fine but will blow up trying to run print(c42)

Actually I tried this example because I couldn’t get the first CRTP example of https://www.fluentcpp.com/2017/09/12/how-to-return-a-smart-pointer-and-use-covariance/ to work. And the problem there was that there are errors in the code on that page. I will comment over there.

Error C2662 ‘double Amount::get_value(void)’: cannot convert ‘this’ pointer from ‘const Amount’ to ‘Amount &’

And another one for Variable.

Am I doing something stupid?