C++ const unsigned char * 到 std::string

const unsigned char * to std::string

提问by LM.

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.

sqlite3_column_text 返回一个 const unsigned char*，如何将其转换为 std::string？我试过 std::string()，但出现错误。

Code:

代码：

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

Error:

错误：

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by Reunanen

You could try:

你可以试试：

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

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

虽然std::string可以有一个带 的构造函数const unsigned char*，但显然它没有。

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

那为什么不呢？你可以看看这个有点相关的问题：为什么 C++ 流使用 char 而不是 unsigned char？

回答by LM.

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

万一你真的想要一串无符号字符，你可以创建自己的类型：

typedef std::basic_string <unsigned char> ustring;

You should then be able to say things like:

然后你应该能够说这样的话：

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

回答by Mike Mueller

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.

人们通常使用 (unsigned char *) 类型的原因是为了表明数据是二进制而不是纯 ASCII 文本。我知道 libxml 会这样做，从它的外观来看，sqlite 也在做同样的事情。

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.

您从 sqlite 调用返回的数据可能是 UTF-8 编码的 Unicode 文本。虽然 reinterpret_cast 可能看起来有效，但如果有人在非纯 ASCII 字段中存储文本，您的程序可能不会表现良好。

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.

std::string 类的设计并没有考虑到 Unicode，所以如果你要求一个字符串的 length()，你会得到字节数，在 UTF-8 中，这不一定是一回事作为字符数。

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.

简短回答：如果您确定数据只是 ASCII，简单的转换可能会起作用。如果它可以是任何 UTF-8 数据，那么您需要以更智能的方式处理编码/解码。

回答by Lyndsey Ferguson

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.

我不熟悉 sqlite3_column_text，但您可能想要做的一件事是，当您调用 std:string 构造函数时，您需要强制转换为 (const char*)。我相信它应该有一个该类型的构造函数。

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.

然而，奇怪的是这个 sqlite 函数返回一个无符号字符*，它返回一个 Pascal 字符串（第一个字符是字符串的长度）？如果是这样，那么您必须使用字节和长度创建 std::string。

回答by user88637

try:

尝试：

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

回答by Kasprzol

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

您不能std::string从 c构造 a onst unsigned char*- 您必须const char*先将其转换为：

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

回答by Raoul Supercopter

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

如果 temp_doc.uuid 是 std::string 尝试：

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

回答by still_learning

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

我不是专家，但这里的这个例子似乎更简单：

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

回答by Erich Kuester

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 vectorwhich I modified to

一个古老但重要的问题，如果您必须在 unsigned char 序列中保留完整信息。在我看来，reinterpret_cast 并非如此。我在将字符串转换为向量下找到了一个有趣的解决方案 ，我将其修改为

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

由于我正在为 gtkmm 编程，因此您可以使用以下方法实现转换为 Glib::ustring

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