CODEWITHSUNDEEP
c++functiontemplates
Ameer khan
Ameer khan

Reputation: 49

Why won't this template function compile C++

I saw this code in the internet:

template <class T, class S>
    T f(T& a, T& b, S c) {
    T t1, t2, t3;
    if (a==b) return t1;
    if (c>0) return t2;
    return t3;
}

Followed by these calls:

f('c', 5, 5); // why this doesn't compile?

f(3, 4, 6.0);  // why does this compile?

The first one doesn't compile, while the second does.. Why?

Upvotes: 0

Views: 69

Answers (1)

AnT stands with Russia
AnT stands with Russia

Reputation: 320381

In C++ (as opposed to C) 'c' is a char.

Your code does not compile because template argument deduction for parameter T fails - it is ambiguous. In f('c', 5, 5) call the first function argument says it should be char, the second says it should be int. How did you expect the compiler to resolve this ambiguity?

If you want T == int you can specify the first template argument explicitly 

f<int>('c', 5, 5);

or you can use an explicit cast 

f((int) 'c', 5, 5);

Of course, as it has already been noted, even if you get through the template argument deduction issues, you still won't be able to initialize non-constant lvalue references with rvalues. Both of your calls are invalid too, for this latter reason.

Upvotes: 4

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