About

propertypp adds Objective-C-like properties to C++ classes. Reading and writing properties uses the same syntax as reading and writing public member variables, but getters and setters get called behind the scenes. For example, an int property p of an object o can be read like this:

int v = o.p;

But what actually happens is this:

int v = o.getp();

Likewise, writing to p uses the following syntax:

o.p = 5;

But what actually takes place is:

o.setp(5);

Keep reading for instructions of how to use propertypp in your projects.

Install

propertypp comes in a single header file, propertypp.h. Just copy it into your code base.

Example code

#include <iostream>
#include "propertypp.h"

class Example
{
    public:
        Example();
        PROPERTYPP_DECLARE(int, n, n_) // declare property n of type int
};

Example::Example()
{
    PROPERTYPP_INIT(n); // initialize property n
}

PROPERTYPP_SYNTHESIZE_GETTER(Example, int, n, n_) // synthesize getter

void setn(int value) // custom setter
{
    if (value >= 0)
        n_ = value;
}

int main()
{
    Example e;

    std::cout << e.n << std::endl; // should print 0

    e.n = 5;
    std::cout << e.n << std::endl; // should print 5

    e.n = -1;
    std::cout << e.n << std::endl; // should print 5

    return 0;
}

Use

propertypp uses a number of macros to add and configure properties in C++ classes.

Declaring properties

To declare a property for a class, use the PROPERTYPP_DECLARE macro.

class Example
{
    public:
        Example();
        PROPERTYPP_DECLARE(int, n, n_)
};

The arguments to PROPERTYPP_DECLARE are the type of the property (int in the example), the name of the property (n in the example) and the name of the member variable that will hold the value of the property (n_ in the example). Notice that you don't have to declare the member variable, because the PROPERTYPP_DECLARE macro does that for you.

IMPORTANT: properties are always public, even if you call the PROPERTYPP_DECLARE macro in private: or protected: scope.

IMPORTANT: after you declare a property, you are left in public: scope.

Defining properties

The PROPERTYPP_DECLARE macro only declares property-related functionality for the class. In order to make properties work, their accessor methods i.e. their getters and setters must be defined.

There are two options for defining property accessor methods. One is to use the PROPERTYPP_SYNTHESIZE* macros to generate them. You can also write your own accessor methods.

Synthesizing getters and setters

The macro PROPERTYPP_SYNTHESIZE generates a default getter and setter for a property:

PROPERTYPP_SYNTHESIZE(Example, int, n, n_)

The arguments to PROPERTYPP_SYNTHESIZE are the name of the class that contains the property, the type of the property, the name of the property and the name of the member variable that stores the value of the property.

If you only want to synthesize the getter or the setter and write the other accessor method yourself, you can use the PROPERTYPP_SYNTHESIZE_GETTER or PROPERTYPP_SYNTHESIZE_SETTER macros, respectively. The arguments to those are exactly the same PROPERTYPP_SYNTHESIZE takes.

IMPORTANT: don't call the PROPERTYPP_SYTHESIZE* macros inside a class declaration.

Writing your own accessor methods

If you want to write the code for an accessor method your self, you have to write the getter and/or setter with the exact following signatures:

Getter:

type get<name>() const;

Setter:

void set<name>(type) const;

Where name is the name of the property as declared with the PROPERTYPP_DECLARE macro and type is the type of the property. For example, the accessor methods for the n property in the Example class above could be written as follows:

int getn() const
{
    return n_;
}

void setn(int val)
{
    n_ = val;
}

Notice that the property name appears as-is on the function names i.e. no camel case. If you define accessor with a signature different than that presented, your code will not build.

Initializing properties

Properties must be initialized with the PROPERTYPP_INIT macro before they can be used.

Example::Example()
{
    PROPERTYPP_INIT(n);
}

The PROPERTYPP_INIT macro takes a sole argument, which is the name of the property. Property values are always initialized to 0.

IMPORTANT: the constructor of a class is likely the best place to initialize any properties it has, but that is not required. However, using a property before it is initialized will probably cause your program to crash.

Using properties

As explained in the About section, properties can be read and written just as if they were a member variable of their class. For example:

Example e;
e.n = 12;
int v = e.n; // v should be 12

Copying properties

If a class containing properties is to support copying via a copy constructor and/or an assignment operator, those methods must be written i.e. you can't rely on compiler-generated copy constructors and assignment operators for proper property copying.

To copy a property, use the PROPERTYPP_COPY macro:

Example::Example(const Example& rhs)
{
    PROPERTYPP_COPY(n, rhs);
}

The arguments to PROPERTYPP_COPY are the name of the property and the name of the variable where the object containing the property is copying its values from.

Failing to copy properties with the PROPERTYPP_COPY macro will lead to property-sharing behavior i.e. setting a property on an object created by copying another will cause that property to be set on the original object also.

Drawbacks

• propertypp currently only works with built-in and pointer property types.
• The behind-the-scenes calls to getters and setters have the cost of virtual function calls.

License

propertypp - Objective-C-like properties for C++ classes.

Copyright (c) 2011 Cesar L. B. Silveira

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.