Type Deduction & Failures

Basic Deduction

template<typename T>
T add(T a, T b) { return a + b; }

int r = add(3, 5); // T := int

References and const

template<typename T>
void modify(T& v) { v *= 2; }  // T := int; parameter type is T&

template<typename T>
void print(const T& v) { /* ... */ }  // T := int; const applies in param

Explicit Arguments

short s{42};
auto a = add(s, s);     // T := short
auto b = add<int>(s,s); // force T := int (converts before call)

When Deduction Fails

• No parameters to deduce from:

template<typename T> T make() { return T{}; }
// auto v = make(); // error
auto v = make<int>();   // OK

• Conflicting parameter types:

template<typename T> T pick(T a, T b) { return a; }
// pick(3, 4.5); // error: T cannot be both int and double