How can I pass a vector variable to a function?

Question

How can we pass vector variables to a function? I have a vector of char* and a function which will take a char * as an argument. How can I pass the vector variable to this function?

Answer 1

If you have a function taking a char* argument, then you can only pass one of the char* in the vector. For example:

std::vector<char*> v;
char buf[] = "hello world";
v.push_back(buf);
the_function(v[0]);

If you want to call the function on each member in the vector, just loop:

for (std::vector<char*>::iterator i = v.begin(); i != v.end(); ++i)
    the_function(*i);

EDIT: based on your comment below, you actually want to write a function that accepts the vector as an argument... try:

void the_function(const std::vector<char*>& v)
{
    // can access v in here, e.g. to print...
    std::cout << "[ (" << v.size() << ") ";
    for (std::vector<char*>::iterator i = v.begin(); i != v.end(); ++i)
         std::cout << *i << ' ';
    std::cout << " ]";
}

If you have an existing function that you want to call, and you don't want to change its argument list...

void TV_ttf_add_row(const char*, const char*, const void*);

...then, say you know the vector will have enough elements:

assert(v.size() >= 3); // optional check...
TV_ttf_add_row(v[0], v[1], v[2]);

or

if (v.size() >= 3)
    TV_ttf_add_row(v[0], v[1], v[2]);

or, if you want an exception thrown if there aren't enough elements in v, then...

try
{
    TV_ttf_add_row(v.at(0), v.at(1), v.at(2));
}
catch (const std::exception& e)
{
    std::cerr << "caught exception: " << e.what() << '\n';
}

(the try/catch block doesn't have to surround the single function call - just as long as the v.at( ) calls are somewhere inside the try block or a function directly or indirectly called from inside the block).

Answer 2

You can also access the vector elements using its index:

void f(char* s)
{
    // do something with s
}

std::vector<char*> char_vect;
size_t len = char_vect.size();
for (size_t i=0; i<len; ++i)
    f(char_vect[i]);

Answer 3

If you need to pass the contents of a std::vector to a function that takes a C array, you can take the address of the first element of the vector:

void someCfunc(char *);

std::vector<char *> somevec;
// set up the vector ... then:
someCfunc(&somevec[0]);

HOWEVER - be careful you don't get the C string convention mixed up here! A std::vector will not append a '\0' to the end of the string automatically; you must do that manually. If it's just a byte buffer, you'll need to pass the vector's size separately. And if you're receiving data back into the vector, the C function cannot expand the vector on its own, so it's up to you to ensure the vector is large enough beforehand.

Note also that this applies only to std::vector and not to other STL containers.