Why Can't C Deduce Template Arguments in Non-Deducible Contexts?

Why Can't C Deduce T in Non-Deducible Contexts?

In the given code snippet, a template struct TMap is defined with a nested type Type that represents the specialization of std::map using the template argument T. However, the compiler fails to deduce T in the function test.

Non-Deducible Context

The error occurs because the code calls test with an argument that has the type typename TMap::Type&, where T is not explicitly specified. This situation is referred to as a non-deducible context.

Ambiguity in Type Inference

In a non-deducible context, the compiler is unable to infer the template argument because the type of the argument does not uniquely determine T. For example, consider the following scenario:

template <>
struct TMap<SomeType> {
  typedef std::map<double, double> Type;
};

template <>
struct TMap<OtherType> {
  typedef std::map<double, double> Type;
};

In this case, both TMap::Type and TMap::Type are identical to std::map. Therefore, the compiler cannot deduce whether T is SomeType or OtherType.

Consequences

The limitation of non-deducible contexts has important implications:

• It prevents the compiler from implicitly specializing templates based on the types of arguments passed to functions.
• It requires explicit template argument deduction or explicit template argument specification when calling functions with generic types as arguments.

Mitigation

To mitigate this limitation, one can employ techniques such as template template arguments or C 20's std::is_same_v to manually deduce template arguments or provide explicit template parameter lists.

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