Why does std::string(“\\x00”) report length of 0?

问题:

I have a function which needs to encode strings, which needs to be able to accept 0x00 as a valid 'byte'. My program needs to check the length of the string, however if I pass in "\x00" to std::string the length() method returns 0.

How can I get the actual length even if the string is a single null character?

回答1:

You're passing in an empty string. Use std::string(1, '\0') instead.

Or std::string{ '\0' } (thanks, @zett42)

回答2:

std::string is perfectly capable of storing nulls. However, you have to be wary, as const char* is not, and you very briefly construct a const char*, from which you create the std::string.

std::string a("\x00");

This creates a constant C string containing only the null character, followed by a null terminator. But C strings don't know how long they are; so the string thinks it runs until the first null terminator, which is the first character. Hence, a zero-length string is created.

std::string b("");
b.push_back('\0');

std::string is null-clean. Characters (\0) can be the zero byte freely as well. So, here, there is nothing stopping us from correctly reading the data structure. The length of b will be 1.

In general, you need to avoid constructing C strings containing null characters. If you read the input from a file directly into std::string or make sure to push the characters one at a time, you can get the result you want. If you really need a constant string with null characters, consider using some other sentinel character instead of \0 and then (if you really need it) replace those characters with '\0' after loading into std::string.

回答3:

With C++14, you can use a string literal operator to store strings with null bytes:

using namespace std::string_literals;

std::string a = "\0"s;
std::string aa = "\0\0"s; // two null bytes are supported too