Why Do I Get 'Invalid Use of Incomplete Type' When Partially Specializing a Template Function?

Invalid Use of Incomplete Type in Partial Template Specialization

When attempting to partially specialize a template function, you may encounter the error "invalid use of incomplete type." This error typically arises when the partially specialized template is not fully defined.

Consider this example code:

template <typename S, typename T>
struct foo {
    void bar();
};

template <typename T>
void foo<int, T>::bar() {
    // Function definition
}

Here, the partial specialization for foo is missing a definition for bar. This results in the compiler error, as the type foo becomes incomplete.

To resolve this, you must fully define the partial specialization template. For example:

template <typename S, typename T>
struct foo {
    void bar();
};

template <>
void foo<int, double>::bar() {
    // Function definition
}

In this modified code, the partial specialization template is fully defined, including the definition for bar. This removes the error and allows the code to compile successfully.

Note that partial specialization cannot be applied to functions alone. To achieve this functionality, you must partially specialize the entire template class. In cases of large templated classes, you may need to consider workarounds such as nested templates or template inheritance.

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