Why Does Template Argument Deduction Fail with Implicit Type Conversions in C ?

Template Argument Deduction and Implicit Type Conversion

In C , template argument deduction is a powerful feature that automatically determines the types of template parameters based on the caller's arguments. However, certain types of conversions are not considered during deduction, including user-defined conversions.

Problem with Implicit Conversion

Consider the following code snippet:

<code class="cpp">template<typename Dtype>
class Scalar{
    Scalar(Dtype v) : value_(v){}
private:
    Dtype value_;
};</code>

The Scalar class represents a simple value type. Now, consider the following template function:

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

This function takes an int and a Scalar object as arguments.

In the following main function, we attempt to call func with an int and an int value:

<code class="cpp">int main(){
    int a = 1;
    func(a, 2); // ERROR: template argument deduction fails
    return 0;
}</code>

But this results in a compilation error, stating that the template argument deduction failed. This is because the compiler cannot automatically convert the int value 2 to a Scalar object. User-defined conversions, like Scalar::Scalar(Dtype value), are not taken into account during template argument deduction.

Possible Solutions

To fix this issue, you have several options:

• Explicit Conversion at Caller Site:

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

  This manually converts the int value to a Scalar object before passing it to func.

• Deduction Guide: (C 17 only)
  Add a deduction guide for Scalar:

  <code class="cpp">template<typename T>
  Scalar(T v) -> Scalar<T>;</code>

  This tells the compiler to prefer this deduction guide when deducing the type of Scalar from the caller's argument, allowing you to call func as:

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

• Explicit Instantiation:
  You can explicitly instantiate func for a specific type:

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

  This forces the compiler to instantiate the function with Dtype = int, circumventing the need for template argument deduction.

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