Problem with declaring a dynamic array of pointers to strings

Question

While performing a practice assignment online I came across a problem which I am not able to solve.

The user has to a number(number of sentences he will be entering) and then proceed to enter the sentences one by one, which are to be stored as strings(by the way, declaring a dynamic array of pointers is mandatory). However, since the number of sentences is not a priori deducible, I know that the size of the array of pointers actually is the number of sentences but I can't figure out how to declare a dynamic array of pointers to strings.

Using something I had already known beforehand, I figured out how to do the same but with arrays of characters, not arrays of strings. The line that declared a dynamic array of pointers to dynamic arrays of chars looked like this:

char **ptr=new char*[n] {};

So with my understanding, this creates a pointer ptr which points to a dynamic array of pointers, the elements of which each point to one array of characters. I want to do something similar now, where the result should be that ptr is a pointer to a dynamic array of pointers, the elements of which each point to a string.

Can anyone help out? I'd be thankful!

Are you willing to use a std::string, or do you want to do this in C with char pointers? — doctorlove
– doctorlove, Commented May 7, 2019 at 10:07
Yes, as I mentioned I have to use std::string for this one. — l0ner9
– l0ner9, Commented May 7, 2019 at 10:10
Gotcha and @doctorlove has got a pretty good approach to that one. If you need more strings, you will have to write a realloc scheme where you basically declare a newset of pointers (with double the number), copy old to new, delete[] old and then set old=newset and keep going. — David C. Rankin
– David C. Rankin, Commented May 7, 2019 at 10:34

Walter · Accepted Answer · 2019-05-07 10:41:11Z

2

I think what you are looking for is something like

std::size_t num;
std::cout << "enter the number of sentences\n";
std::cin  >> num;
std::string *sentences = new std::string[num];
for(std::size_t i=0; i!=num; ++i) {
    std::cout << "enter the " << (i+1) << "th sentence\n";
    std::cin  >> sentences[i];
}
/* 
    ... (do something with the sentences, accessing them as sentences[i])
*/
delete[] sentences;     // free the memory

Note that this style of coding is highly discouraged. The problem is the need to manage the allocated memory: avoid memory leaks and dangling pointers (including exception safety). The correct approach is to use a container or smart pointer. For example:

std::size_t num;
std::cout << "enter the number of sentences\n";
std::cin  >> num;
std::vector<std::string> sentences{num};
for(std::size_t i=0; i!=num; ++i) {
    std::cout << "enter the " << (i+1) << "th sentence\n";
    std::cin  >> sentences[i];
}
/* 
    ... (do something with the sentences, accessing them as sentences[i])
*/

or

std::size_t num;
std::cout << "enter the number of sentences\n";
std::cin  >> num;
std::unique_ptr<std::string[]> sentences{new std::string[num]};
for(std::size_t i=0; i!=num; ++i) {
    std::cout << "enter the " << (i+1) << "th sentence\n";
    std::cin  >> sentences[i];
}
/* 
    ... (do something with the sentences, accessing them as sentences[i])
*/

when in both cases you don't have to worry about calling delete: the allocated memory will be automatically deleted (even if an exception occurs).

answered May 7, 2019 at 10:41

Walter

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2 Comments

l0ner9 Over a year ago

Thank you very much, that seemed to work. I have one more question though(if you have the time to reply). I think I may have come up with this on my own, but what confuses me is std::string *sentences = new std::string[num];. sentences is a pointer to strings, but new std::string[num] is something I haven't seen before. Isn't std::string internally a dynamically allocated array? What would new do in conjunction with string? Once again I am sorry for my ignorance, I am really new to this. Thank you for your immense help!

Walter Over a year ago

@I0ner9 If this answer was helpful, please upvote it and/or accept it. As to your additional question. This is standard operator new[] syntax, as in int *ptr = new int[42];, only that instead of int you allocate std::string. Yes, internally std::string may also use dynamic allocation to store long strings, but that does not matter here: the new operator only allocates the memory actually needed by the std::string, not the additional memory that the strings then may allocate.

doctorlove · Accepted Answer · 2019-05-07 10:19:00Z

2

You can avoid pointers completely and use

std::vector<std::string> input;

A std::array needs to know the size at compile time, and you learn this at runtime. The vector works like an array but can have items push_backed at runtime.

You could declare pointer to some strings, using n once you know it:

std::string * pInputs = new std::string[n];

but it's easier to use the vector. Each pInput will be a string, as with the std::vector version.

answered May 7, 2019 at 10:19

doctorlove

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