const unsigned char * to std::string

问题:

sqlite3_column_text returns a const unsigned char*, how do I convert this to a std::string? I've tried std::string(), but I get an error.

Code:

temp_doc.uuid = std::string(sqlite3_column_text(this->stmts.read_documents, 0));

Error:

1>.\storage_manager.cpp(109) : error C2440: '<function-style-cast>' : cannot convert from 'const unsigned char *' to 'std::string'
1>        No constructor could take the source type, or constructor overload resolution was ambiguous

回答1:

You could try:

temp_doc.uuid = std::string(reinterpret_cast<const char*>(
      sqlite3_column_text(this->stmts.read_documents, 0)
  ));

While std::string could have a constructor that takes const unsigned char*, apparently it does not.

Why not, then? You could have a look at this somewhat related question: Why do C++ streams use char instead of unsigned char?

回答2:

On the off-chance you actually want a string of unsigned characters, you could create your own type:

typedef std::basic_string <unsigned char> ustring;

You should then be able to say things like:

ustring s = sqlite3_column_text(this->stmts.read_documents, 0);

回答3:

The reason people typically use an (unsigned char *) type is to indicate that the data is binary and not plain ASCII text. I know libxml does this, and from the looks of it, sqlite is doing the same thing.

The data you're getting back from the sqlite call is probably UTF-8 encoded Unicode text. While a reinterpret_cast may appear to work, if someone ever stores text in the field that is not plain ASCII, your program probably won't be well-behaved.

The std::string class isn't designed with Unicode in mind, so if you ask for the length() of a string, you'll get the number of bytes, which, in UTF-8, is not necessarily the same thing as the number of characters.

Short answer: the simple cast may work, if you're certain the data is just ASCII. If it can be any UTF-8 data, then you need to handle encoding/decoding in a smarter way.

回答4:

I'm not familiar with sqlite3_column_text, but one thing you may want to do is when you call the std:string constructor, you'll want to cast to (const char*). I believe that it should have a constructor for that type.

However, it is odd that this sqlite function is return an unsigned char*, is it returning a Pascal string (first char is the length of the string)? If so, then you'll have to create the std::string with the bytes and the length.

回答5:

try:

temp_doc.uuid = std::string(reinterpret_cast<const char*>(sqlite3_column_text(this->stmts.read_documents, 0)));

回答6:

if temp_doc.uuid is a std::string try :

temp_doc.uuid = static_cast<const char*>(sqlite3_column_text(this->stmts.read_documents, 0));

回答7:

You can't construct a std::string from const unsigned char* -- you have to cast it to const char* first:

temp_doc.uuid = std::string( reinterpret_cast< const char* >(
  sqlite3_column_text(this->stmts.read_documents, 0) ) );

回答8:

I'm no expert but this example here seems much simpler:

string name = (const char*) (sqlite3_column_text(res, 0));

回答9:

An old but important question, if you have to preserve the full information in the unsigned char sequence. In my opinion that is with reinterpret_cast not the case. I found an interesting solution under converting string to vector which I modified to

basic_string<unsigned char> temp = sqlite3_column_text(stmt, 0);
string firstItem( temp.begin(), temp.end() );

Since I am programming for gtkmm, you can realize the conversion into a Glib::ustring with

basic_string<unsigned char> temp = sqlite3_column_text(stmt, 0);
Glib::ustring firstItem = string( temp.begin(), temp.end() );