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Outer class containing inner class - instantiate the inner class as a member of the outer class
#ifndef TERMINALLOG_HH
#define TERMINALLOG_HH

#include <string>
#include <sstream>
#include <iostream>

class Terminallog {
public:

    Terminallog();
    virtual ~Terminallog();

    class Warn {
    public:
        Warn();

    private:
        bool lineended;
    };
    friend class Terminallog::Warn;

protected:

private:
    Warn warn;

};

// stripped code

 Terminallog::Terminallog() {
     Warn this->warn();
 }

Terminallog::Warn::Warn() {
    this->lineended = true;
}

//stripped code

g++ src/terminallog.cc inc/terminallog.hh -o test -Wall -Werror
In file included from src/terminallog.cc:8:
src/../inc/terminallog.hh:56: error: declaration of ‘Terminallog::Warn Terminallog::warn’
src/../inc/terminallog.hh:24: error: conflicts with previous declaration ‘void Terminallog::warn(std::string)’

Terminallog::Terminallog() : warn()
{
   // other constructor code
}


// note that `Warn::Warn()` is invoked implicitly on `wake`, so

TerminalLog::Terminallog() {}

// would be equivalent