how to insert vector element into link with cURL c_str() function in C ?-CodePudding

I'm working on a c code that fetches a variable from a text file and adds it to a fixed url, similarly to the following example:

int x = numbers[n];
string url = "http://example.com/"   x;
curl_easy_setopt(curl, CURLOPT_URL, url.c_str());

but I'm getting this error message after compiling

Protocol "tp" not supported or disabled in libcurl

How can I fix that?

CodePudding user response：

int x = numbers[n];
string url = "http://example.com/"   x;

In c/c there is pointer arithmetic.

const char* ptr = "abcde";
string s1 = ptr   0; // "abcde";
string s2 = ptr   1; // "bcde";
string s3 = ptr   2; // "cde";
...

So your string is wrong.

Please check to_string() and const char* to string on web.

CodePudding user response：

You are adding an int to a const char* pointer (that was decayed from a string literal of type const char[20]). That will offset the pointer by however many elements the int indicates. Which, in your case, appears to be 2, which is why CURL thinks the URL begins with tp: instead of http:.

Your code is basically the equivalent of this:

const char strliteral[] = "http://example.com/";
...
int x = numbers[n];
const char *p = strliteral;
string url = p   x; // ie: &p[x]

h t t p : / / e x a m p l e . c o m /
^   ^
|   |
p   p 2

To fix that, you can use std::to_string() in C 11 and later to convert the int to a string, eg:

string url = "http://example.com/"   to_string(x);

Or, you can use a std::ostringstream (in all C versions), eg:

ostringstream oss;
oss << "http://example.com/" << x;
string url = oss.str();

Or, you can use std::format() in C 20 and later (you can use the {fmt} library in earlier versions), eg:

string url = format("http://example.com/{}", x);
// or:
string url = format("{}{}", "http://example.com/", x);