How to have two functions that call each other C++

What you want is forward declaration. In your case:

void funcB(string str);

void funcA(string str)
{
    size_t f = str.find("if");
    if(f!=string::npos)
    {
        funcB(str);        //obfuscate if-loop
    }
}

void funcB(string str)
{
     //obfuscate if loop
     funcA(body_of_if_loop);     //to check if there is a nested if-loop
}

A forward declaration would work:

void funcB(string str); 

void funcA(string str)
{
    size_t f = str.find("if");
    if(f!=string::npos)
    {
        funcB(str);        //obfuscate if-loop
    }
}

void funcB(string str)
{
     //obfuscate if loop
     funcA(body_of_if_loop);     //to check if there is a nested if-loop
}