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

This is the documentation for an old version of Boost. Click here to view this page for the latest version.
Library Documentation Index

Safe Numerics




This type contains the functions to return a safe type corresponding to the C++ type resulting from a given arithmetic operation.

Usage of this policy with safe types will produce the exact same arithmetic results that using normal unsafe integer types will. Hence this policy is suitable as a drop-in replacement for these unsafe types. Its main function is to trap incorrect arithmetic results when using C++ for integer arithmetic.

Model of


As an example of how this works consider C++ rules from section 5 of the standard - "usual arithmetic conversions".

void int f(int x, int y){
    auto z = x + y; // z will be of type "int"
    return z;

According to these rules, z will be of type int. Depending on the values of x and y, z may or may not contain the correct arithmetic result of the operation x + y.

using safe_int = safe<int, native>;
void int f(safe_int x, safe_int y){
    auto z = x + y; // z will be of type "safe_int"
    return z;
Example of use

The following example illustrates the native type being passed as a template parameter for the type safe<int>. This example is slightly contrived in that safe<int> has native as its default promotion parameter so explicitly using native is not necessary.

#include <cassert>
#include <boost/numeric/safe_numerics/safe_integer.hpp> 
#include <boost/numeric/safe_numerics/native.hpp> 
int main(){
    using namespace boost::numeric;
    // use native promotion policy where C++ standard arithmetic 
    // might lead to incorrect results
    using safe_int8 = safe<std::int8_t, native>;
        safe_int8 x = 127;
        safe_int8 y = 2;
        safe_int8 z;
        // rather than producing an invalid result an exception is thrown
        z = x + y;
        assert(false); // never arrive here
    catch(std::exception & e){
        // which we can catch here
        std::cout << e.what() << std::endl;

    // When result is an int, C++ promotion rules guarentee 
    // that there will be no incorrect result.
    // In such cases, there is no runtime overhead from using safe types.
    safe_int8 x = 127;
    safe_int8 y = 2;
    safe<int, native> z; // z can now hold the result of the addition of any two 8 bit numbers
    z = x + y; // is guaranteed correct without any runtime overhead or exception.

    return 0;

See Chapter 5, Expressions, C++ Standard


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