question about pointers and overloading (.append() doesn't work in one line)

In my program I would like to manipulate boost::filesystem::path elements of a vector in a for loop.

typedef vector<fs::path> path_vec;
path_vec pv;

for (auto it = pv.cbegin(), end = pv.cend(); it != end; ++it)

What I would like to do is to add a string to the end of the path.

If I do it like this, it works fine:

stringstream image_file_0001;
image_file_0001 << it->string() << "/x_alpha0001.png";
p1 = image_file_0001.str();

If I do it like this, it works fine too:

string a = it->string();
string b = a.append("/prx");

But if I try to do it in one line

string c = it->string().append("/prx");

or

string d = (it->string()).append("/prx");

it gives compile err开发者_开发技巧ors:

7 overloads have no legal conversion for 'this' pointer

I think it must be my lack of knowledge about how to use pointers, or is it something to do with the boost::filesystem::path .string() function?

OK, the thing I would like to do is to create a directory with "/prx" appended to the origianl path in *it. Can I do it in one line?

boost::filesystem::createdirectory ( something here );

What seems to be my problem is that I don't understand that why would .append() modify the original string. Isn't it a function which returns an other string, which I can use freely, while just reading the original string?

fs::path.string() returns const& so you cannot append anything to it, first you need to make a copy

but why do you do this at all? there's a obvious way to append nested path:

path / nested_path

EDIT:

typedef vector<fs::path> path_vec;
path_vec pv;

for (auto it = pv.begin(), end = pv.end(); it != end; ++it)
    *it /= "prx";

or to create directories instead of modifying vector values, replace the last line by:

fs::create_directory(*it / "prx");

it->string() is probably constant. Why don't you say:

const std::string a = it->string() + "/prx";