observer

A C++17 templated observer mechanism as a header-only library that is inspired by Qt's signals and slots.

Features

Highlights of this library are:

Defining the subject's notification values by variadic template parameters.
Connect all kinds of callables to a subject like member functions, lambdas, functors and free functions.
Connected callables can accept less parameters than the subject provides.

Examples

This is a short introduction about the features and usage of this library.
See observer_demo.cpp for a full reference.

Example 1

This example shows the basic usage of the library.

void hello_function()
{
    std::cout << "Hello World!" << std::endl;
}

int main( int /* argc */, char * /* argv */[] )
{
    pg::observer_owner owner;
    pg::subject<>      hello_subject;
    
    owner.connect( hello_subject, hello_function );
    
    hello_subject.notify();
    
    return 0;
}

The output is:

Hello World!

So, what happened here?

There are two objects declared in the main function:

owner manages lifetime of the connection between subjects and observers.
hello_subject which will be used to emit notifications to observers that are connected.

Next, the owner connects hello_function observer function to the hello_subject.

When hello_subject does a notify then all observer functions that are connected are called.

Example 2

This example shows some more advanced features of the library:

Objects can inherit from pg::observer_owner.
Connecting member functions to a subject.
Connecting lambdas to a subject.
Connecting observer functions that accepts less arguments than the subject's notify passes on.

struct bar : public pg::observer_owner
{
    bar( pg::subject< std::string, int >& foo )
    {
        connect( foo, this, &bar::print );
        connect( foo, []( const std::string &str )
        {
            std::cout << "Hello " << str << "!" << std::endl;
        } );
    }

    void print( const std::string &str, int i )
    {
        std::cout << str << i << std::endl;
    }
};

int main( int /* argc */, char * /* argv */[] )
{
    pg::subject< std::string, int > foo;
    bar                             b( foo );

    foo.notify( "PG", 1003 );

    return 0;
}

The output is:

PG1003
Hello PG!

Why C++17?

One of the key parts in this library uses std::apply which is introduced with C++17. My limited brain capacity was not sufficient to implement a std::apply-like function.

yanzipei / observer