Why Doesn't C Support Template Argument Inference for Class Constructors?

Template Inference for Constructor Arguments: Exploring the Limitations

In C , template parameters can be inferred from function arguments, allowing for concise and intuitive code. However, this same functionality is not available for class constructors, raising questions about the underlying rationale.

The key distinction lies in the potential for multiple points of entry into a class. Unlike functions with a single, defined entry point, constructors can be supplemented by copy constructors and assignment operators. If template inference were allowed for constructors, ambiguity would arise when constructing objects via these alternative entry points.

Consider the following example:

template <typename obj>
class Variable {
    obj data;
public:
    Variable(obj d) { data = d; }
};

int main() {
    int num = 2;
    Variable var(num); // interpreted as Variable<int> var(num)
    Variable other;  // ambiguous: which template parameter type is inferred?
    other = var;     // constructs via assignment operator, potentially causing inference issues
    return 0;
}</int></typename>

In this scenario, inferring the template parameter type from the constructor argument for var is straightforward. However, when attempting to use the default constructor for other and subsequently assigning var to it, it becomes unclear what template parameter type should be inferred. The same issue arises if copy ctors for var and other are defined, leading to potential confusion and errors.

Furthermore, there are instances where inferring the template parameter type could be undesirable. Consider a generic function that accepts a class template as an argument. If inference were allowed, it would be difficult to explicitly specify the template parameter type, potentially restricting the flexibility of the function.

In conclusion, while template inference for class constructors might initially appear appealing, the challenges and ambiguity it introduces outweigh its potential benefits. The existing approach, which allows for explicit specification of template parameters, provides greater clarity and avoids potential inference issues.

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