Is there a TryParse equivalent in C++ (gcc)? - c++

Question

Is there a equivalent of TryParse in C++(gcc) ?
I would like to parse a string which may contain (+31321) and store it as long. I know phone numbers are stored as strings and string matched but for my needs , I would like to store them as long and sometimes they might contain pluses (+). What would parse it in C++?

Answer 1

The trouble with strtoul() and family there is no real way to test for failure.
If it fails to parse then it returns 0 without setting errno (which is only set on overflow).
boost lexical cast
#include <boost/lexical_cast.hpp>
int main()
{
try
{
long x = boost::lexical_cast<long>("+1234");
std::cout << "X is " << x << "\n";
}
catch(...)
{
std::cout << "Failed\n";
}
}
Doing it using streams
int main()
{
try
{
std::stringstream stream("+1234");
long x;
char test;
if ((!(stream >> x)) || (stream >> test))
{
// You should test that the stream into x worked.
// You should also test that there is nothing left in the stream
// Above: if (stream >> test) is good then there was content left after the long
// This is an indication that the value you were parsing is not a number.
throw std::runtime_error("Failed");
}
std::cout << "X is " << x << "\n";
}
catch(...)
{
std::cout << "Failed\n";
}
}
Using scanf:
int main()
{
try
{
char integer[] = "+1234";
long x;
int len;
if (sscanf(integer, "%ld%n", &x, &len) != 1 || (len != strlen(integer)))
{
// Check the scanf worked.
// Also check the scanf() read everything from the string.
// If there was anything left it indicates a failure.
throw std::runtime_error("Failed");
}
std::cout << "X is " << x << "\n";
}
catch(...)
{
std::cout << "Failed\n";
}
}

Answer 2

+31321 can be parsed as a long with the usual stream extraction operators.
#include <iostream>
#include <sstream>
int main()
{
std::istringstream s("+31321");
long n;
s >> n;
std::cout << n << '\n';
}
demo: http://ideone.com/4rmlp
Although parsing an actual phone number (with parentheses, dashes, extensions, etc) may not be as simple.

Answer 3

In fact, before convert the string into a number, the numbers should be "normalized" to a common format. This requires removing all symbols, and replace them with proper representation.
But you must be very care about representing telephone numbers (that are NOT numbers: they are not subjected to regular arithmetic) as string: Number beginning with one or more zeroes are not the same as the ones with the zeroes removed:
00 is the typical replacement for +, but a number without the 00 (or +) in front should be prefixed by 00c where c is the country code.
You need to do some pre-processing to come to a uniform string representation, before converting, otherwise you risk to "alias" different things.

Answer 4

the input extraction operator >> (i hope it's an acceptable name) applies and returns a stream&, that has a bool operator, meaning the extraction has been succesfully attempted. For instance, from the Cubbi answer:
...
std::istringstream s("+31321");
long n;
if (s >> n)
std::cout << n << '\n';
....
This will succed, of course, given the appropriate content of s.
Somewhat different (easier but not type safe) also the scanf family has practical tools available in c++ as well c. You could of course write the example this way:
...
long n;
if (sscanf("+31321", "%d", &n) == 1)
std::cout << n << '\n';
...
A subset of regular expressions make this rather powerful: for instance to match a comma separed multi fields with left space trimming:
if (sscanf("a,b,c", " [^,], [^,], [^,]", a,b,c) == 3) ...