Incovenience of avoiding duplicate code due to an if statement c++11 - c++

Question

I want to avoid duplicated code in this usecase
class A {
protected:
virtual void A1(const void* const s, const std::streamsize n) const;
inline void A2(const void* const s, const std::streamsize n) const;
};
class B : public A {
private:
const char *a;
void B1(const char *b) {
if (!b) {
return;
}
if (a < b) {
A1(a, b-a);
}
}
void B2(const char *b) {
if (!b) {
return;
}
if (a < b) {
A2(a, b-a);
};
}
};
So, as you can see above in both B1() and B2() there is duplicate code (that check for b) except for the call inside that if (note that the if condition is the same). I think this ifmakes somehow inconvenient to extract a new method, but also I think it can be done using lambdas and/or templates. There is no point of interest on how A1() and A2() are implemented for this usecase.
My question: What is the best and simplest way to avoid this duplication of code ?

Answer 1

You can write a function that accepts pointer to member to be executed
class B : public A {
private:
const char *a;
using F = void(A::*)(const void* const, const std::streamsize) const;
void RunFun(F f, const char *b) {
if (!b) {
return;
}
if (a < b) {
(this->*f)(a, b-a);
}
}
void B1(const char *b) {
RunFun(&B::A1,b);
}
void B2(const char *b) {
RunFun(&B::A2,b);
}
};

Answer 2

Another (simplified) example, using lambda and std::function
#include <cstring>
#include <iostream>
#include <functional>
struct A
{
virtual void A1 (char const * const b)
{ std::cout << b << "\n- A1 call" << std::endl; }
void A2 (char const * const b)
{ std::cout << b << "\n- A2 call" << std::endl; }
};
struct B : public A
{
const char * a;
std::function<void(char const * const, void(A::*)(char const * const))>
funcA { [this](char const * const b, void(A::*f)(char const * const))
{ if ( b && std::strlen(b) ) (this->*f)(b); } };
void B1 (char const * b)
{ funcA(b, &A::A1); }
void B2 (char const * b)
{ funcA(b, &A::A2); }
};
int main ()
{
B b;
b.B1("- B1 call");
b.B2("- B2 call");
}