boost::lexical_cast cannot handle negative numbers?

Question

This short C++ program behaves in a way which baffles me:

#include <cassert>
#include <iostream>
#include <string>
#include <boost/lexical_cast.hpp>

int main(void) {
    signed char c = -2;
    assert(c == -2);
    c = boost::lexical_cast<signed char>(std::string("-2"));
    std::cout << c << "\n";
}

Using g++ 5.2.1 and boost-1.58.0, I get:

terminate called after throwing an instance of 'boost::exception_detail::clone_impl >' what(): bad lexical cast: source type value could not be interpreted as target

Why can't Boost cast from string "-2" to signed char given that the value -2 is representable by this type?

Because `lexical_cast` behaves as if the conversion is done via a stringstream, and `operator>>` for `signed char` reads characters, not numbers. — T.C., Dec 19 '15 at 23:17
Never mind, I can do `boost::lexical_cast(str)` and then convert to `signed char` checking the range... — quant_dev, Dec 19 '15 at 23:21

score 3 · Answer 1 · edited Dec 20 '15 at 13:22

3

The solution is to use Boost:

#include <boost/lexical_cast.hpp>
#include <boost/numeric/conversion/cast.hpp>
int tmp = boost::lexical_cast<int>(std::string("-2"));
char c = boost::numeric_cast<signed char>(tmp);

edited Dec 20 '15 at 13:22

T.C.

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answered Dec 20 '15 at 13:09

quant_dev

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boost::lexical_cast cannot handle negative numbers?

1 Answers1