Matching a string at the beginning of a inputstream - c++

Question

I have implemented a simple inputstream manipulator to match the next n chars in an inputstream against a given string. However, I am not sure if this is the best way to do this. Any hints?
class MatchString {
private:
std::string mString;
public:
MatchString(const std::string &str) {
mString = str;
}
std::istream& operator()(std::istream& is) const {
// Allocate a string buffer, ...
char *buffer = new char[mString.length()];
// ... read next n chars into the buffer ...
is.read(buffer, mString.length());
// ... and compare them with given string.
if(strncmp(buffer, mString.c_str(), mString.length())) {
throw MismatchException(mString);
}
delete[] buffer;
return is;
}
};
inline MatchString match(const std::string &str) {
return MatchString(str);
}
inline std::istream& operator>>(std::istream& is, const MatchString& matchStr) {
return matchStr(is);
}
EDIT:
A solution consuming the matched chars could be implemented based on the suggestion of user673679:
class MatchString {
...
std::istream& operator()(std::istream& is) const {
// Match the next n chars.
std::for_each(mString.begin(), mString.end(),
[&](const char c) {
if(is.get() != c) {
throw MismatchException(mString);
}
});
return is;
}
};
How would I implement this if I don't want to consume the chars?
EDIT II:
Here another solution mentioned by fjardon:
class MatchString {
...
std::istream& operator()(std::istream& is) const {
// Match the next n chars.
if(std::mismatch(mString.begin(), mString.end(),
std::istreambuf_iterator<char>(is)).first != mString.end()) {
throw MismatchException(mString);
}
return is;
}
};
EDIT III:
Finally got a working function, that will revert consumption, if string doesn't match:
class MatchString {
...
std::istream& operator()(std::istream& is) const {
// Match the next n chars.
std::streampos oldPos = is.tellg();
if(std::mismatch(mString.begin(), mString.end(),
std::istreambuf_iterator<char>(is)).first != mString.end()) {
is.seekg(oldPos);
throw MismatchException(mString);
}
return is;
}
};

Answer 1

Instead of allocating and copying the whole string from the stream, you could just check one character at a time and avoid allocating the buffer completely:
#include <iostream>
#include <sstream>
#include <string>
auto mString = std::string("foobar");
std::istream& match(std::istream& is) {
for (auto c : mString)
if (c != is.get())
throw std::runtime_error("nope");
return is;
}
int main()
{
auto input = "foobarbaz";
auto stream = std::istringstream(input);
match(stream);
std::cout << "done!" << std::endl;
}
You should also add error checking for is.get() (or .read() in your original code).