c++: new multi dimensional array of strings [duplicate]

Question

Possible Duplicate:
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
– jonathan topf, Commented Jun 9, 2011 at 12:33

Björn Pollex · Accepted Answer · 2011-06-09 12:13:27Z

2

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.

answered Jun 9, 2011 at 12:13

Björn Pollex

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Kiril Kirov · Accepted Answer · 2011-06-09 12:12:48Z

1

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.

answered Jun 9, 2011 at 12:12

Kiril Kirov

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Tony The Lion · Accepted Answer · 2011-06-09 12:12:35Z

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 9, 2011 at 12:12

Tony The Lion

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1 Comment

Beta Over a year ago

I think you have the dimensions in the wrong order. It should be 3 first, then numberOfCommands.

Andrew White · Accepted Answer · 2011-06-09 13:01:05Z

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 9, 2011 at 13:01

answered Jun 9, 2011 at 12:12

Andrew White

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2 Comments

Beta Over a year ago

This doesn't work. It just isn't how C++ handles arrays.

Andrew White Over a year ago

@Beta: sorry, forgot the lhs array part; this should work now.

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