Why Does Implicit Type Conversion Fail in Template Argument Deduction?

Implicit Type Conversion and Template Deduction

In C , template argument deduction is a mechanism that allows type parameters to be automatically inferred based on the function arguments. However, there are limitations to implicit type conversion in template deduction.

Consider the following code snippet:

<code class="cpp">#include<iostream>
using namespace std;

template<typename Dtype>
class Scalar{
public:
  Scalar(Dtype v) : value_(v){}
private:
  Dtype value_;
};

template<typename Dtype>
void func(int a, Scalar<Dtype> b){ 
  cout << "ok" <<endl;
}

int main(){
  int a = 1;
  func(a, 2); // error: no matching function for call to ‘func(int&, int)’
  return 0;
}

Here, the goal is to call the func() template function with an integer a and an implicitly converted Scalar object 2. However, this results in a compilation error. Why is this?

Understanding the Error

The compiler fails to perform template argument deduction because:

• Implicit type conversion is not considered during template argument deduction.
• The conversion from int to Scalar is a user-defined conversion, which is not recognized by the compiler during deduction.

Solution Options

To resolve this issue, several approaches can be taken:

• Explicit Conversion at Caller Site:
  Convert the argument manually at the caller site:

  <code class="cpp">func(a, Scalar<int>{2});</code>

• Deduction Guide (C 17 ):
  Define a deduction guide for Scalar and call func() as:

  <code class="cpp">func(a, Scalar{2});</code>

• Explicit Template Instantiation (Only if Scalar Constructor is not Explicit):
  Explicitly instantiate the func() template for the desired type parameter:

  <code class="cpp">func<int>(a, 2); </code>

Conclusion

Template argument deduction in C is a powerful mechanism, but it has limitations when dealing with user-defined conversions. By utilizing the discussed solutions, developers can ensure that template functions are called with the intended type parameters.

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