Using new with an array of constant length arrays

I'm having trouble getting to grips with the syntax of new in c++. I come from a C background.

The thing is I have a an array

Code:

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float (*arr)[3]

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so a pointer to an array of 3 floats. I now wish to allocate memory to the pointer arr so that it can hold an array of float-triples. I know I could do it by making arr of type float ** instead and then allocate in a loop, but since I know the length of each element there must be an easier way that clearly shows my intention.

I tried with

Code:

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arr=new float[3] [N]

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but this refuses to compile.

With your C background, you should have known that arr is an array of 3 pointers to float, not a pointer.

Create a struct instead. Or even a class with overloaded [] operator. Or just use std::array.

If you just want a pointer to an array of 3 floats then you don't specify the size in the declaration. You just declare a pointer and specify the size where you do allocation.
In C you'd do:

Code:

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float *arr = malloc(sizeof(float) * 3);

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and in C++:

Code:

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float *arr = new float[3];

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It is unusual to allocate such a small fixed-size buffer in real-world code however, you'd typically declare a local array, or just use a std::vector.
Welcome to the happy land of C++!

Quote:

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Originally Posted by iMalc

If you just want a pointer to an array of 3 floats then you don't specify the size in the declaration.

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What he actually wants to achieve is:

Quote:

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an array of float-triples

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Like:

Code:

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std::vector< std::array<float, 3> >

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As Laserlight pointed out there was indeed a mistake in the first post, I have corrected it now (happens when you are in a hurry).

Thank you for your answers. I fond that using

Code:

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arr=new float[N] [3]

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seems to do what I want. The suggestion to use std::vector and std::array seems very interesting as well, but I have to interface with some C libraries so I need to be able access the data as an C array of float-triples. Is it possible to access the underlying consecutive memory block as a normal C array by making a vector of std::arrays (or a std::array of std::arrays)?

You got the sizes in the wrong order when you called new.

Code:

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    float  (*arr)[3];
    arr = new float[10][3];
    delete [ ] arr;

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Edit:
Heh, looks like you figured it out :)

Indeed :) The syntax seems a bit backwards to me however. I figured the syntax was "new type amount", and since the type I want is float[3] and the amount is N I seemed to get it wrong.

Quote:

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Originally Posted by cppfreak

Is it possible to access the underlying consecutive memory block as a normal C array by making a vector of std::arrays (or a std::array of std::arrays)?

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Vector and Array are contiguous in memory and you can obtain address of the first element with: &vec[0] or &arr[0], although I am not sure if this is guaranted for 2 dimensions (I do not know whether std::array is allowed to hold additional information), like:

Code:

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std::vector< std::array<float, 3> > vec;

&vec[0][0] + 3 == &vec[1][0] // satisfied?

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