Why Does Type Deduction Fail for My TMap Template in This Function Call?

Type Deduction Failure in TMap Template

In response to the query regarding a deduction error when invoking the test function, the issue resides in the non-deducible context in which the T template parameter is instantiated.

Within the test function, the compiler attempts to deduce the type T based on the type of the parameter tmap_. However, since there could potentially be multiple specializations of the TMap template for different types (e.g., TMap and TMap), the compiler cannot uniquely determine the type of T solely by examining the type of tmap_.

Consider a hypothetical specialization:

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

In this scenario, TMap::Type would be equivalent to std::map. However, if another specialization exists for OtherType, which also defines its type as std::map, the compiler cannot infer which type T corresponds to when confronted with the std::map type of tmap_.

The lack of information available to the compiler to establish a unique correspondence between the type of tmap_ and the type of T leads to the reported deduction error. To resolve this issue, the type T must be explicitly specified when invoking the test function, ensuring the correct template specialization is selected.

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