I stumbled upon Stack Overflow question Memory leak with std::string when using std::list<std::string>, and one of the comments says this:
Stop using
new
so much. I can't see any reason you used new anywhere you did. You can create objects by value in C++ and it's one of the huge advantages to using the language. You do not have to allocate everything on the heap. Stop thinking like a Java programmer.
I'm not really sure what he means by that.
Why should objects be created by value in C++ as often as possible, and what difference does it make internally? Did I misinterpret the answer?
Best Answer
There are two widely-used memory allocation techniques: automatic allocation and dynamic allocation. Commonly, there is a corresponding region of memory for each: the stack and the heap.
Stack
The stack always allocates memory in a sequential fashion. It can do so because it requires you to release the memory in the reverse order (First-In, Last-Out: FILO). This is the memory allocation technique for local variables in many programming languages. It is very, very fast because it requires minimal bookkeeping and the next address to allocate is implicit.
In C++, this is called automatic storage because the storage is claimed automatically at the end of scope. As soon as execution of current code block (delimited using
{}
) is completed, memory for all variables in that block is automatically collected. This is also the moment where destructors are invoked to clean up resources.Heap
The heap allows for a more flexible memory allocation mode. Bookkeeping is more complex and allocation is slower. Because there is no implicit release point, you must release the memory manually, using
delete
ordelete[]
(free
in C). However, the absence of an implicit release point is the key to the heap's flexibility.Reasons to use dynamic allocation
Even if using the heap is slower and potentially leads to memory leaks or memory fragmentation, there are perfectly good use cases for dynamic allocation, as it's less limited.
Two key reasons to use dynamic allocation:
You don't know how much memory you need at compile time. For instance, when reading a text file into a string, you usually don't know what size the file has, so you can't decide how much memory to allocate until you run the program.
You want to allocate memory which will persist after leaving the current block. For instance, you may want to write a function
string readfile(string path)
that returns the contents of a file. In this case, even if the stack could hold the entire file contents, you could not return from a function and keep the allocated memory block.Why dynamic allocation is often unnecessary
In C++ there's a neat construct called a destructor. This mechanism allows you to manage resources by aligning the lifetime of the resource with the lifetime of a variable. This technique is called RAII and is the distinguishing point of C++. It "wraps" resources into objects.
std::string
is a perfect example. This snippet:actually allocates a variable amount of memory. The
std::string
object allocates memory using the heap and releases it in its destructor. In this case, you did not need to manually manage any resources and still got the benefits of dynamic memory allocation.In particular, it implies that in this snippet:
there is unneeded dynamic memory allocation. The program requires more typing (!) and introduces the risk of forgetting to deallocate the memory. It does this with no apparent benefit.
Why you should use automatic storage as often as possible
Basically, the last paragraph sums it up. Using automatic storage as often as possible makes your programs:
Bonus points
In the referenced question, there are additional concerns. In particular, the following class:
Is actually a lot more risky to use than the following one:
The reason is that
std::string
properly defines a copy constructor. Consider the following program:Using the original version, this program will likely crash, as it uses
delete
on the same string twice. Using the modified version, eachLine
instance will own its own string instance, each with its own memory and both will be released at the end of the program.Other notes
Extensive use of RAII is considered a best practice in C++ because of all the reasons above. However, there is an additional benefit which is not immediately obvious. Basically, it's better than the sum of its parts. The whole mechanism composes. It scales.
If you use the
Line
class as a building block:Then
allocates four
std::string
instances, fourLine
instances, oneTable
instance and all the string's contents and everything is freed automagically.