C++ why can you initialize an empty string array?

Question

This is a "how does it work" question. Based on my understanding, you have to initialize a non-dynamic array with a constant number of elements (int intarr[5]) or your array will write over blocks of memory that might be allocated for something else.

So why is it that you can initialize a string array (string strArray[]) without any elements?

Example:

#include <iostream>
#include <string>
using namespace std;

int main()
{
string s[] = {"hi", "there"};

cout << s[0] << s[1];
cin >> s[10]; //why is this ok?
cout << s[10];

return 0;
}

I'm not sure if I understand your question... string strArray[]; is not correct either — Natan Streppel
– Natan Streppel, Commented Sep 17, 2013 at 20:24
Please update your question to show small self-contained program containing a declaration that illustrates what you're asking about. — Keith Thompson
– Keith Thompson, Commented Sep 17, 2013 at 20:25
Oh so adding on to the array is not ok? I was informed that it was. (Which is why im so confused as to why it would not cause an issue) — cplusJones
– cplusJones, Commented Sep 17, 2013 at 20:32

nos · Accepted Answer · 2013-09-17 20:29:22Z

5

As a feature C++ (and C) allows you to declare an array without specifying the number of elements if you specify an initializer for it. The compiler will infer the number of elements that's needed.

So for

   int intarr[] = { 1,2,3,4,5};

The compiler will see that the array need to have room for 5 ints, and this is will be exactly the same as if you stated:

int intarr[5] = {1,2,3,4,5};

A string array is just the same;

 string s[] = {"hi", "there"};

is the same as if you wrote

 string s[2] = {"hi", "there"};

Your code has a serious bug though;

string s[] = {"hi", "there"};

cout << s[0] << s[1];
cin >> s[10]; //why is this ok?

cin >> s[10]; is absolutely NOT ok. The array s has only 2 elements, and even if the compiler does not generate an error, you cannot use s[10] Doing so results in undefined behavior - so it could appear to work, or it could crash, or it could do anything.

answered Sep 17, 2013 at 20:29

nos

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2 Comments

cplusJones Over a year ago

Ok thank you, I was just misinformed. This really clears it up.

lapk Over a year ago

I would also mention that you need to specify all dimensions (in multidimensional arrays), except for the first one, which compiler can deduce.

Mark Lakata · Accepted Answer · 2013-09-17 20:27:53Z

0

You can use string strArray[] in 2 ways

string strArray[] = {"apple","banana"};

or

void function(string strArray[]) {
    ...
}

In the first case, the size of the array is automatically determined by the number of initializers (2). In the second case, the size of the array is not needed because it is a formal argument.

Other than that, you must declare a size to the array. It doesn't matter if it is based on string or int.

If you are writing to s[10] in your example but only have 2 elements, you are writing to unallocated memory and the program behavior is undefined. It may not crash, but later on something bad will happen.

answered Sep 17, 2013 at 20:27

Mark Lakata

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3 Comments

jrok Over a year ago

void function(string strArray[]); is completely equivalent to void function(string* strArray);. That is, it's not an array at all.

nos Over a year ago

@MarkLakata in a parameter list, they really are exactly the same.

Mark Lakata Over a year ago

@jrok and nos - I stand corrected and learned something. There is a difference if you have a multidimensional array though, but for one dimension, there no difference.

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