Why do public members of a base class become invisible when inheriting a class template, and what are the solutions to this problem?

Avoiding Public Member Invisibility and Code Bloat in Inherited Class Templates

Object-oriented programming relies on the inheritance of protected and public class members. In non-templated environments, this inheritance is straightforward. However, when introducing class templates, a fundamental issue arises: public members of the base class become invisible to the derived class template.

The Problem

Consider the following example:

<code class="cpp">class CBase
{
public:
    int Fn1(void) { ... }
};

class CDerived : public CBase
{
public:
    int FnSum(void) { ... CBase::Fn1(); ... }
};</code>

When templating this code, the Fn1() member of CBase disappears from the perspective of CDerived:

<code class="cpp">template<unsigned int BYTES>
class CBase
{
public:
    int Fn1(void) { ... }
};

template<unsigned int BYTES>
class CDerived : public CBase<BYTES>
{
public:
    int FnSum(void) { ... Fn1(); ... } // Error: Fn1 not found!
};</code>

Solutions

Various solutions exist to address this issue:

Solution 1 (Verbose Prefixing):

<code class="cpp">int FnSum(void) {
    return CBase<BYTES>::Fn1() + CBase<BYTES>::Fn2() + CBase<BYTES>::Arr[0];
}</code>

However, this approach introduces excessive code bloat when referencing multiple members.

Solution 2 (Verbose "this->" Prefixing):

<code class="cpp">int FnSum(void) {
    return this->Fn1() + this->Fn2() + this->Arr[0];
}</code>

Similar to Solution 1, but slightly less verbose.

Solution 3 (Using Statement):

<code class="cpp">using CBase<BYTES>::Arr;
using CBase<BYTES>::Fn1;
using CBase<BYTES>::Fn2;</code>

This allows direct access to CBase members without additional prefixing.

Solution 4 (Permissive Mode):

<code class="cpp">// Compiler flags: /permissive- (MSVC), -fpermissive (GCC/Cl), -fno-implicit-templates (Clang)</code>

Disables strict C compliance, resolving the issue but may introduce portability concerns.

A Better Solution

While Solution 3 is cleaner than the others, it still requires repetitive "using" statements. Macros can simplify this:

<code class="cpp">#include <boost/preprocessor.hpp>

#define USING_ONE(r, base, member) using base::member;

#define USING_ALL(base, ...) BOOST_PP_SEQ_FOR_EACH(USING_ONE, base, BOOST_PP_VARIADIC_TO_SEQ(__VA_ARGS__))

#define USING_CBASE(param) USING_ALL(CBase<param>, Arr, Fn1, Fn2, Fn3, Fn4, Fn5)</code>

In the derived class:

<code class="cpp">USING_CBASE(BYTES); // Makes all mentioned CBase members directly accessible</code>

This approach significantly reduces code bloat and duplication.

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