Why Does Implicit Type Conversion Fail in Template Deduction?

Implicit Type Conversion in Template Deduction

In C , the implicit type conversion mechanism plays a crucial role in template argument deduction. However, in certain scenarios, it can lead to unexpected behavior, as illustrated by the code snippet below:

<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); // Incorrect conversion
  //int b = 2;
  //func(a, b); // Also incorrect
  return 0;
}

In the provided code, the goal is to invoke the template function func by implicitly converting an int to a Scalar object. However, surprisingly, the code fails to compile due to template argument deduction/substitution failing for the first call to func(a, 2).

The reason behind this failure is that template argument deduction does not consider user-defined conversions. In this scenario, the conversion from int to Scalar is a user-defined conversion, which is not automatically applied during template argument deduction.

To resolve this issue, there are several options:

• Explicit Conversion at Caller Site: Force the conversion by manually providing a Scalar object with the desired value at the call site:

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

• Deduction Guide: Define a deduction guide for Scalar and call func:

  <code class="cpp">func(a, Scalar{2}); // C++17 only</code>

  This approach relies on the default deduction guide, which is sufficient in this case.

• Explicit Instantiation: Explicitly instantiate the func template for the desired type:

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

  This works only if Scalar::Scalar(T) is not explicit.

In conclusion, template argument deduction does not automatically apply user-defined conversions. To use user-defined conversions in template argument deduction, it is necessary to either explicitly convert the argument at the caller site, use a deduction guide if applicable, or explicitly instantiate the template for the desired type.

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