{"id":6972,"date":"2014-07-07T14:41:17","date_gmt":"2014-07-07T13:41:17","guid":{"rendered":"https:\/\/blogs.mentor.com\/colinwalls\/?p=6972"},"modified":"2014-07-07T14:41:17","modified_gmt":"2014-07-07T13:41:17","slug":"uniqueness-of-c-methods-and-class-member-variables","status":"publish","type":"post","link":"https:\/\/blogs.stage.sw.siemens.com\/embedded-software\/2014\/07\/07\/uniqueness-of-c-methods-and-class-member-variables\/","title":{"rendered":"Uniqueness of C++ methods and class member variables"},"content":{"rendered":"

I have made a number of recent postings<\/a> focused on C++ issues, responding to a number of questions. I have a few more planned, but I was intending to give this topic a rest for a while. However, my eye was caught by one question, which I felt had some potential:<\/p>\n

I learned that Objective C objects are not complete copies of the class object. In other words, only the data and certain static structures are independent for each instance of a class. The methods are kept in a central location for all objects to use. Is there any C++ implementation for embedded systems that use this feature?<\/p>\n

This sounds simple, but this question raises quite a few issues …<\/p>\n

First off, Objective C is of limited interest to embedded developers. So a comparison with C++ may be of limited value.<\/p>\n

In C++, there is no such thing as a “class object” really. A class [or a struct] is a “recipe” to make objects. Defining a class\/struct does not create any code or data. Instantiating the class – i.e. defining objects [or variables, if you prefer] – is when real code and data is created\/allocated. It is at that point that some discussion about memory usage makes sense. Here is a simple class definition:<\/p>\n

class example\n{\npublic:\n   int local_variable;\n   static int shared_variable;\n   void out_of_line_function(int p);\n   int inline_function(int x)\n   {\n      local_variable = x;\n      return x;\n   };\n};<\/pre>\n
<\/p>\n
void example::out_of_line_function(int p)\n{\n   shared_variable = p;\n}<\/pre>\n
<\/p>\n
example class_instantiation_1, class_instantiation_1;<\/pre>\n
Here we have a class – example<\/strong> – and an object instantiated from it – class_instantiation_1<\/strong> and class_instantiation_2<\/strong>.<\/p>\n
<\/p>\n
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<\/div>\n
#172515644<\/a> \/ gettyimages.com<\/a><\/div>\n<\/div>\n
Now we can look at each member variable and function [method] individually:<\/p>\n