Memory Allocation Exception in Constructor

Question

I have a constructor that allocates several blocks of memory using the new operator.

X::X() {
    a = new int[100];
    b = new char[100];
    c = new float[100];
}

My question is, if the allocation of c fails, and the constructor throws an exception, will the memory for a and b be automatically freed?

No. This is what the technique known as [RAII](https://en.wikipedia.org/wiki/Resource_Acquisition_Is_Initialization) is useful for. If you want a class for managing a dynamic array, it is called [`std::vector`](http://en.cppreference.com/w/cpp/container/vector). Don't try to write one yourself. — BoBTFish, Feb 25 '16 at 14:15
Required reading: [Why should C++ programmers minimize use of 'new'?](http://stackoverflow.com/questions/6500313/why-should-c-programmers-minimize-use-of-new) — Cody Gray - on strike, Feb 25 '16 at 14:18
Besides the use of `std::vector`, if the size is a compile-time fixed constant, and the memory don't have to allocated from the heap, consider the use of [`std::array`](http://en.cppreference.com/w/cpp/container/array) instead. — Some programmer dude, Feb 25 '16 at 14:19
The question should show the types of `a`, `b` and `c`, and you should also clarify whether you mean to ask if the memory will be freed. — M.M, Feb 26 '16 at 06:36

Bathsheba · Accepted Answer · 2016-02-26T06:22:46.667

14

The memory to which a and b point would not be automatically freed. Every new[] must be balanced explicitly with a delete[].

Even if your destructor performed the deletion (assuming a, b, and c are class members), then you'd still leak memory. That's because the destructor wouldn't be called in this case since the object failed to construct.

Using std::vectors would obviate these problems.

edited Feb 26 '16 at 06:22

answered Feb 25 '16 at 14:17

Bathsheba

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technically, `a` and `b` are automatically destroyed. But destroying a pointer doesn't do anything. – Mooing Duck Feb 25 '16 at 20:24

Pete Becker · Answer 2 · 2016-02-25T17:57:00.487

9

a, b, and c will all be destroyed. Depending on what types they are, that might or might not release the memory. If they are pointers their destructors don't do anything, and the memory leaks. If they are some sort of smart pointer, presumably their destructors will release the memory.

edited Feb 25 '16 at 17:57

answered Feb 25 '16 at 14:23

Pete Becker

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Won't all three data members be destroyed? Their constructors have all finished running by the time the X constructor begins running. – templatetypedef Feb 25 '16 at 17:18
(Note that the destructors for the data members, if they're raw pointers, won't deallocate the pointers.) – templatetypedef Feb 25 '16 at 17:18

score 3 · Answer 3 · edited May 23 '17 at 10:28

3

No they won't. This is why you need to learn about RAII and in particular, containers and smart pointers.

In your case, you can use std::vector<T> instead of new T[100], for example.

edited May 23 '17 at 10:28

Community

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answered Feb 25 '16 at 14:16

Simple

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score 1 · Answer 4 · answered Feb 25 '16 at 14:28

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Variable a and b will not be automatically destroyed for sure. In your case you must use this std::vector. This is because whenever we use a new[] operator for that we need to explicitly define a delete[].

answered Feb 25 '16 at 14:28

Chandra Bhushan

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Memory Allocation Exception in Constructor

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