c++ - 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 technical decision pass vector-in-vector objects pointers because don't want them copied , passed function. it's because vectors hold more 1000 strings.

but don't understand why compile error @ 2 lines 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

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

would valid code if used references, which should have.

as 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 proper operator associativity).

without it, pointer canceled out [index] (ub if index != 0), luckily, doesn't compile, because end assigning std::vector<std::string> std::string.

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