CODEWITHSUNDEEP
c++pointers
TheTobruk
TheTobruk

Reputation: 17

Assinging pointer to string variable fails

match give_word(
    const vector< vector<string> > * source_words,
    const vector< vector<string> > * target_words, ...)
{
    string synonym1, synonym2;
    ...
    // choose synonyms (if available)
    synonym1=source_words[index][rand()%source_words[index].size()];
    synonym2=target_words[index][rand()%target_words[index].size()];
    ...

I've made a technical decision to pass all my vector-in-vector objects as pointers because I don't want them to be copied and passed to the function. It's because those vectors usually hold more than 1000 strings.

But what I don't understand is why I get the compile error at those two lines about the assignment sign (=)

synonym1=source_words[index][rand()%source_words[index].size()];
synonym2=target_words[index][rand()%target_words[index].size()];

It says this: No operator "=" matches these operands

Upvotes: 0

Views: 45

Answers (2)

LogicStuff
LogicStuff

Reputation: 19607

synonym1 = source_words[index][rand() % source_words[index].size()];
synonym2 = target_words[index][rand() % target_words[index].size()];

would be a valid code if you used references, which you should have.

As for pointers, you'll need this:

synonym1 = (*source_words)[index][rand() % (*source_words)[index].size()];
synonym2 = (*target_words)[index][rand() % (*target_words)[index].size()];

That's called dereferencing (parentheses for proper operator associativity).

Without it, the pointer is canceled out by [index] (UB if index != 0), but luckily, it doesn't compile, because you end up with assigning std::vector<std::string> to std::string.

Similarly, with source_words[index].size(), you're getting the size of the std::vector<std::vector<std::string>> and not std::vector<std::string>.

Upvotes: 2

Jts
Jts

Reputation: 3527

Every time there's a chance to use references over pointers, go ahead and do that.

Note that if at some point, you must use a pointer to a vector, you can always use the at function like

synonym1 = source_words->at(index)[rand() % source_words->at(index).size()];
synonym2 = target_words->at(index)[rand() % target_words->at(index).size()];

I know some people prefer to call like myfunc(&vector) (to somehow "make it obvious" that the function being called is going to modify the vector), so that's a way to work with pointers to vectors, or dereference the pointer as LogicStuff pointed out and then use the [] operator.

A very important fact though, is that the at function checks whenever the "index" is valid or not, while the operator [] doesn't.

Upvotes: 1

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