std::basic_istream<CharT,Traits>::peek
Min standard notice:
Behaves as UnformattedInputFunction. After constructing and testing the sentry object, reads the next character from the input stream without extracting it.
# Declarations
int_type peek();
# Return value
If good() == true, returns the next character as obtained by rdbuf()->sgetc().
# Example
#include <iostream>
#include <sstream>
int main()
{
std::istringstream s1("Hello, world.");
char c1 = s1.peek();
char c2 = s1.get();
std::cout << "Peeked: " << c1 << " got: " << c2 << '\n';
}