Why Do I Get 'Undefined Reference to' Errors for Template Class Constructors, and How Can I Fix Them?

"Undefined Reference to" Template Class Constructor [Duplicate]

The compiler error "undefined reference to..." for template class constructors signifies a lack of explicit instantiation instructions for the specific template classes used in the program. This issue arises because the compiler doesn't automatically compile template code until it's explicitly required.

Solution 1: Explicit Template Instantiation

Append explicit template instantiation statements to the end of the implementation file (cola.cpp in this case):

template class cola<float>;
template class cola<string>;

Additionally, include the following lines in nodo_colaypila.cpp:

template class nodo_colaypila<float>;
template class nodo_colaypila<std::string>;

This forces the compiler to compile the required template classes.

Solution 2: Include Implementation in Header File

Move the implementation code from cola.cpp and nodo_colaypila.cpp into cola.h and nodo_colaypila.h, respectively. This ensures the availability of function definitions in all translation units using the template classes.

Pros and Cons of Each Solution:

• Solution 1:

  • Pro: More efficient as it only compiles the necessary template instances.
  • Con: Requires manual instantiation for each desired template class.

• Solution 2:

  • Pro: More convenient and flexible, as it allows for future template class instantiations.
  • Con: Potentially slower compilation due to duplicate code generation for each translation unit.

Other Typos in the Code:

• Mismatched pointer syntax in the cola.h declaration:

nodo_colaypila<T><T>* ult, pri;

should be:

nodo_colaypila<T>* ult, *pri;

• Missing #endif at the end of nodo_colaypila.h.
• Default parameter for nodo_colaypila constructor should be defined in the header file, not the implementation file.

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