Boost C++ Libraries of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

Library Documentation Index

Safe Numerics



Template Parameters
Model of
Valid Expressions
Example of use
See Also


checked_result is a special kind of variant class designed to hold the result of some operation. It can hold either the result of the operation or information on why the operation failed to produce a valid result. It is similar to other types proposed for and/or included to the C++ standard library or Boost such as expected, variant, optional and outcome. In some circumstances it may be referred to as a "monad".

  • All instances of checked_result<R> are immutable. That is, once constructed, they cannot be altered.

  • There is no default constructor.

  • checked_result<R> is never empty.

  • Binary operations supported by type R are guaranteed to be supported by checked_result<R>.

  • Binary operations can be invoked on a pair of checked_result<R> instances if and only if the underlying type (R) is identical for both instances. They will return a value of type checked_result<R>.

  • Unary operations can be invoked on checked_result<R> instances. They will return a value of type checked_result<R>.

  • Comparison operations will return a boost::logic::tribool. Other binary operations will a value of the same type as the arguments.

Think of checked<R> as an "extended" version of R which can hold all the values that R can hold in addition other "special values". For example, consider checked<int>.


Symbol Description
R Underlying type
r An instance of type R
c, c1, c2 an instance of checked_result<R>
t an instance of checked_result<T> for some type T not necessarily the same as R
e An instance of type safe_numerics_error
msg An instance of type const char *
OS A type convertible to std::basic_ostream
os An instance of type convertible to std::basic_ostream

Template Parameters

R must model the type requirements of Numeric

Parameter Description
R Underlying type

Model of


Valid Expressions

All expressions are constexpr.

Expression Return Type Semantics
checked_result(r) checked_result<R> constructor with valid instance of R
checked_result<R>(t) checked_result<R> constructor with checked_result<T> where T is not R. T must be convertible to R.
checked_result(e, msg) checked_result<R> constructor with error information
static_cast<R>(c) R extract wrapped value - compile time error if not possible
static_cast<safe_numerics_error>(c) safe_numerics_error extract wrapped value - may return safe_numerics_error::success if there is no error
static_cast<const char *>(c) const char * returns pointer to the included error message
c.exception() bool true if checked_result contains an error condition.
c1 < c2
c1 >= c2
c1 > c2
c1 <= c2
c1 == c2
c1 != c2
boost::logic::tribool compare the wrapped values of two checked_result instances. If the values are such that the result of such a comparison cannot be reasonably defined, The result of the comparison is boost::logic::tribool::indeterminant.
c1 + c2
c1 - c2
c1 * c2
c1 / c2
c1 % c2
c1 | c2
c1 & c2
c1 ^ c2
c1 << c2
c1 >> c2
checked_result<R> returns a new instance of checked_result<R>.
os << c
OS writes result to output stream. If the result is an error it writes the string corresponding to the error message. Otherwise, it writes the numeric value resulting from the operation. Returns reference to output stream.

Example of use

//  Copyright (c) 2018 Robert Ramey
// Distributed under the Boost Software License, Version 1.0. (See
// accompanying file LICENSE_1_0.txt or copy at

#include <iostream>

#include <boost/safe_numerics/checked_result.hpp>
#include <boost/safe_numerics/checked_result_operations.hpp>

int main(){
    using ext_uint = boost::safe_numerics::checked_result<unsigned int>;
    const ext_uint x{4};
    const ext_uint y{3};

    // operation is a success!
    std::cout << "success! x - y = " << x - y;

    // subtraction would result in -1, and invalid result for an unsigned value
    std::cout << "problem: y - x = " << y - x;

    const ext_uint z = y - x;
    std::cout << "z = " << z;
    // sum of two negative overflows is a negative overflow.
    std::cout << "z + z" << z + z;

    return 0;

See Also



#include <boost/numeric/safe_numerics/checked_result.hpp>

#include <boost/numeric/safe_numerics/checked_result_operations.hpp>