Conditional template

Question

I have a class with template Inside this class there is a variable T * Variable; and variable string str; and there is function with part of the code:

if(std::is_same<T, string>::value){
    *Variable = str;
    return 0;
}

Naturally despite above asignment wont work on different variables due to condition before, compiler moans on compile time. I was thinking about writing conditional template function with two template arguments to put it in place of above asignment:

Assign(Variable, &str);

And before the class:

template <typename T, typename U>
conditional <T, U> void Assign(T* to, U* from){
    ;
}
conditional <string, string> void Assign(T* to, U* from){
    *to = *from;
}

So the compiler would choose specialized one for my needed string. How to formulate this? Can this be formulated just for variable types or do I need a third argument? Or maybe there can be some sort of template condition for just primary code inside function?

Can you use `C++17`? [`if constexpr`](https://en.cppreference.com/w/cpp/language/if) — tkausl, Jan 24 '20 at 17:10
Looks like an [xy problem](https://meta.stackexchange.com/a/66378) to me. Why do you need to assign a value conditionally based on type? Would it be possible to overload the function that this snippet comes from? — eike, Jan 24 '20 at 17:23

score 2 · Answer 1 · answered Jan 24 '20 at 17:31

2

Just use template specialization:

template<typename T>
class Foo {
    T * Variable;

public:
    explicit Foo(T *p) : Variable{p}
    {}

    void Assign(const std::string& str) {
        std::cout << str << " No op\n";
    }
};

template<>
void Foo<std::string>::Assign(const std::string& str) {
    *Variable = str;
}

https://wandbox.org/permlink/Oi1bl8LCQQQ2LyJC

This will work with quite old C++ version (at least C++03).

answered Jan 24 '20 at 17:31

Marek R

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So if I understand correctly: You have here heritage of function that you are overloading using template for just string? And you are list initializing varieable with p inside constructor? Hmm so this overloaded function should be overloaded outside of a Foo class. I need to use it inside this class. – Gen0me Jan 24 '20 at 18:43
@Gen0me This is not inheritance, but [template specialization](https://en.cppreference.com/w/cpp/language/template_specialization). The method is not overloaded, but the implementation is changed for the template parameter `std::string`. – eike Jan 24 '20 at 19:10
I solved the problem, but Im still curious: Will this method work for function calls inside a class – Gen0me Jan 24 '20 at 19:25

score 2 · Answer 2 · answered Jan 24 '20 at 18:08

2

If you have access to C++17, then simply use if constexpr:

if constexpr (std::is_same<T, string>::value) {
    *Variable = str;
    return 0;
}

answered Jan 24 '20 at 18:08

Guillaume Racicot

39,621
9
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I am getting error: expected '(' before 'constexpr' I already updated to c++ 17 compilation – Gen0me Jan 24 '20 at 18:15
@Gen0me that kind of error usually shows up when C++17 is either disabled or unsupported. What is your compiler version and what are your flags? – Guillaume Racicot Jan 24 '20 at 18:22
TDM64 g++, -fpermissive – Gen0me Jan 24 '20 at 18:27
@Gen0me C++17 is enabled with the `-std=c++17` flag. You'll need GCC 7 or better for that. I'm not sure what version you have right now. – Guillaume Racicot Jan 24 '20 at 18:30
Also, you should try to get rid of `-fpemissive`. That flag is not your friend. I'd try to enable it only in needed translation units. – Guillaume Racicot Jan 24 '20 at 18:31
Actually it compiles without -fpermissive – Gen0me Jan 24 '20 at 18:45

score 0 · Answer 3 · answered Jan 24 '20 at 19:20

So I wrote the Assign function before my class with template and template specialization

template<typename T, typename U>
void Assign(T* to, U* from){
    ;
}
template<>
void Assign(string* to, string* from){
    *to = *from;
}

And I changed line *Variable = str; into:

Assign(Variable, &str);

Here is a topic that helped me: Templated Functions.. ERROR: template-id does not match any template declaration

Conditional template

3 Answers3