c++: new multi dimensional array of strings

Question

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C++ multi dimensional array

I'm trying to create a class that creates an arbitrary sized array of strings based on one of the objects constructor arguments.

Here, the code for the object constructor I'm trying so far:

commandSpec::commandSpec(int numberOfCommands)
{
    std::string * commands = new std::string[3][numberOfCommands];
}

I get an error: 'numberOfCommands cannot appear in a constant expression', could someone show me the correct way to specify an array in an object that i dont know the size of until execution.

Thanks, j

so is it bad form to add an array on the heap like that (if i fix the syntax)? I thought that as the size wont change after its created i wouldnt need the featres that a vector would give me, i just need a simple array thats size will be defined at runtime but will still be fixed when it is defined. — jonathan topf, Jun 09 '11 at 12:33

score 2 · Accepted Answer · answered Jun 09 '11 at 12:13

This should probably be implemented as a structure and a vector, like this:

struct command {
    std::string first;
    std::string second;
    std::string third;
};

commandSpec::commandSpec(int numberOfCommands)
{
    std::vector<command> commands(numberOfCommands);
}

Of course, you should choose appropriate names for the members of command.

score 1 · Answer 2 · answered Jun 09 '11 at 12:12

Variable length arrays are allowed only when allocating on heap.

You need to allocate the array in 2 steps - first allocate array with length 3 (from pointers) and then loop through the 3 elements and allocate new string for each.

I'd recommend you to use std::vector instead.

Andrew White · Answer 3 · 2011-06-09T13:01:05.043

0

Invert the order of the dimensions...

commandSpec::commandSpec(int numberOfCommands)
{
    std::string (*commands)[3] = new std::string[numberOfCommands][3];
}

However, I highly recommend you consider using vectors instead.

edited Jun 09 '11 at 13:01

answered Jun 09 '11 at 12:12

Andrew White

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This doesn't work. It just isn't how C++ handles arrays. – Beta Jun 09 '11 at 12:23
@Beta: sorry, forgot the lhs array part; this should work now. – Andrew White Jun 09 '11 at 13:01

score 0 · Answer 4 · answered Jun 09 '11 at 12:12

0

I would use a std::vector instead, makes life easier:

commandSpec::commandSpec(int numberOfCommands)
{
    std::vector<std::vector<std::string>> mystrings(numberOfCommands);
}

answered Jun 09 '11 at 12:12

Tony The Lion

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I think you have the dimensions in the wrong order. It should be 3 first, then numberOfCommands. – Beta Jun 09 '11 at 12:25

c++: new multi dimensional array of strings

4 Answers4