C ++，指针，引用，从一个函数传递到另一个函数，std :: vector，std :: string

Question

i have a specific problem with pointers and references, with std::vector and std::string. Some question are in the following code snipped, some below.

I have basically this code

//first of all: is this function declaration good?
//or shoudl I already pass the std::vector in another way?
//I'm only reading from it
void func(const std::vector<std::string>& vec)
{
    //I need to call otherFunc(void*) with  a single element from vec:
    //otherFunc(void* arg);
    //with arg being a void* to vec[0]
}

My IDE tells me that only &*vec[0] works as parameter for otherFunc , but this doesn't compile...

How is the best way to do these kind of parameter passing?

Answer 1

That is a good declaration, as long as the function is not intended to modify the vector. It's more efficient than passing by value, since it avoids copying the vector - an expensive operation requiring a memory allocation.

However, the other function requires a non-const pointer. How to handle this depends on whether it might modify the data.

If it won't (as you imply when you say "I'm only reading from it") then the options are:

• Change it to otherFunc(void const * arg) to give a stronger guarantee that it won't, or
• Remove the const qualification with a const_cast<void*> when calling it

Note that &*vec[0] won't compile; you want vec[0].c_str() to get a C-compatible pointer to the first string's data, assuming that's what you need.

If it might modify the vector, then you'll have to do something else, since there's no legal way to modify a std::string through a pointer to its data. Probably the best option is to use std::vector<char> rather than std::string , but that depends on exactly what the function does.

Answer 2

Firstly, &*vec[0] is meaningless; you probably mean &vec[0] (ie the address of the first string in your vector).

But even this won't compile because &vec[0] is of type const std::string * . Most importantly, it's const . You could do this:

otherFunc(const_cast<std::string *>(&vec[0]));

BUT!!! Trying to use a const std::string * in the context of a void * sounds like a very bad idea. How could that possibly ever do anything useful?

Answer 3

First of all, since you only read from the vector, passing it as a const reference is a good idea and should work well.

To your second question: it is difficult to say what otherFunc will do to your object, since it expects a void pointer. This is C style and should not be used in C++, there are better and type safe ways to do so.