How can I assign a std::string from a char * [duplicate]

Question

This may be trivial, but I'm new to C++ and an getting confused here.

I have the method:

bool load_database_file( const std::string& filename, std::string& contents ) { std::ifstream is (filename, std::ifstream::binary); if (is) { // get length of file: is.seekg (0, is.end); int length = (int)is.tellg(); is.seekg (0, is.beg); char * buffer = new char [length]; std::cout << "Reading " << length << " characters... "; // read data as a block: is.read (buffer, length); if (is) std::cout << "all characters read successfully."; else std::cout << "error: only " << is.gcount() << " could be read"; is.close(); // ...buffer contains the entire file... std::string str(buffer); contents = str; delete[] buffer; } return true ; }

where I would like to read a file and assign it's contents to contents so that it can be read by the calling function.

My issue is that after this function is run, I see that only the first character of buffer was copied to contents.

How can I copy/convert the entire contents of buffer (a char *) to contents (a std::string).

Did you get the example from here? Please note that in this case your length is one too short to hold a zero-terminated string, and second that you don't zero-terminate your string before assigning it to std::string. — MicroVirus
– MicroVirus, Commented Jul 7, 2014 at 17:53
@MicroVirus I got the example from here: cplusplus.com/reference/istream/istream/read — Brett
– Brett, Commented Jul 7, 2014 at 17:55
Then my remark holds true. David Schwartz's answer circumvents the issue. — MicroVirus
– MicroVirus, Commented Jul 7, 2014 at 17:57
char * buffer = new char [length]; Replace that with this: std::vector<char> buffer(length); — PaulMcKenzie
– PaulMcKenzie, Commented Jul 7, 2014 at 18:01

Soumadeep Saha · Accepted Answer · 2014-07-07 19:56:05Z

 std::string str(buffer);

Should be:

 std::string str(buffer, buffer+length);

Otherwise, how can the constructor know how many bytes to allocate/copy?

By the way, your code is needlessly clumsy. Why not read directly into the string's buffer rather than using a separate buffer that you have to allocate and free just to hold the data before you allocate another buffer?

Because before C++11 there was no guarantee of contiguity in std::string? And, actually, because the OP should be reading in a loop, to ensure he has all the characters. You don't know how many read invocations that'll take. A buffer inside a loop is the conventional way to handle that.

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