For 2021 - online Python 3 training - see ((here)).
Our plans were to retire in summer 2020 and see the world, but Coronavirus has lead us into a lot of lockdown programming in Python 3 and PHP 7. We can now offer tailored online training - small groups, real tutors - works really well for groups of 4 to 14 delegates. Anywhere in the world; course language English.
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Allocation of memory for objects in C++ - Stack v Heap
In C++, memory is allocated to variables on a stack or on a heap. Allocated on the stack, the variable persists until the end of the block in which it's defined. Allocated on the heap, the memory containing the object persists until the end of your program, or until you delete the object.
Stack memory is set up by a simple variable declaration as shown in the example:
Train * serviceStack() {
Train later(3,79);
cout << "Created with capacity of " << later.getCapacity() << endl;
return &later;
}
And heap memory through the use of the new keyword:
Train * serviceHeap() {
Train * early = new Train(2,75);
cout << "Created with capacity of " << early->getCapacity() << endl;
return early;
}
The stack example here is very, very bad indeed as the returned address points to memory on the stack which will be re-used in subsequent code as the program progresses after the return from the function.
Here's code calling our functions above:
Train * first = serviceHeap();
Train * second = serviceStack();
int e = first->getCapacity();
cout << "Capacity of " << e << endl;
int c = second->getCapacity();
cout << "Capacity of " << c << endl;
and the result of running that code:
Created with capacity of 210
Created with capacity of 331
Capacity of 210
Capacity of -678813632
Within the methods used to create the objects, teh references to the getCapacity method both return the correctly calculated value. Once the creating methods have been exited, the object that's on the heap (the first one) continues to exist and function correctly, whereas the object that wason the stack has pobably been overwritten with something else, and results are "random". Run it again:
Created with capacity of 210
Created with capacity of 331
Capacity of 210
Capacity of -395132864
Many modern compilers will flag a warning if they spot you coding in this way:
WomanWithCat:harwell grahamellis$ g++ -o objects_bothways objects_bothways.cpp
objects_bothways.cpp:26:10: warning: address of stack memory associated with local variable
'later' returned [-Wreturn-stack-address]
return &later;
^~~~~
1 warning generated.
WomanWithCat:harwell grahamellis$
General rule:
Stack variables for temportary objects, heap for persistant ones
Source code of full example [here]. Subject covered on our Learning to program in C++ amd C++ programming courses. (written 2015-10-31)
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles C232 - C and C based languages - Defining and using classes in C++ [1925] Introduction to Object Oriented Programming - (2008-12-06) [2577] Complete teaching example - C++, inheritance, polymorphism - (2010-01-15) [2578] Where are your objects stored in C++? - (2010-01-16) [2579] Creating, setting up and using objects in C++ - (2010-01-16) [3250] C++ - how we teach the language and the concepts behind the language - (2011-04-17) [3716] Learning C++ - a design pattern for your first class - (2012-05-02) [3721] Naming blocks of code, structures and Object Orientation - efficient coding in manageable chunks - (2012-05-06) [3810] Reading files, and using factories to create vectors of objects from the data in C++ - (2012-07-21) [3978] Teaching OO - how to avoid lots of window switching early on - (2013-01-17) [4129] Simple OO demonstration in C++, comparison to Python - (2013-07-01) [4372] Template / design pattern for C++ constructor and accessors - (2014-12-29)
Some other Articles
Lua - changes to how integers and floats are handled - 5.2 to 5.3Moderation - and the tendency to over-moderateJapanese and Malaysian food in MelkshamC - why is slow to write and debug) but fast to run?Allocation of memory for objects in C++ - Stack v HeapPerl, PHP, Python, Lua, Tcl, C++, Ruby - final public courses for 2015Formatting and outputting your own classes in C++Left shift operator on an output stream object - C++Hello World in C++ - a first program, with the process explainedVariables, Pointers and References - C and C++
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