Exceptions help you write such safe code.

exception_example.cpp

#include <exception>
#include <stdexcept>
#include <vector>

#include <assert.h>
#include <stddef.h>

// Returns a munged/hash of an int.
// Always returns something different than what is passed in.
int munge(int i){
    // We can't munge zero, it would still be zero, so that's an exception!
    if(i == 0)
        throw std::runtime_error("You're screwed now.");
    return (i >> 2) ^ (i << 7);
}

class frobulator{
    std::vector<int> m_data0, m_data1;
public:
    void add(int i){
        // We always add to both the vectors at once, so it's safe!
        m_data0.push_back(i);
        m_data1.push_back(munge(i));
        
        // Munging should always return different data.
        assert(m_data0.back() != m_data1.back());
    }
    
    void dump(){
        // We know the vectors are the same size, so it's safe!
        for(size_t i = 0; i < m_data0.size(); i++){
            printf("%i -> %i\n", m_data0[i], m_data1[i]);
        }
    }
};

int main(int argc, char **argv){
    frobulator f;
    for(size_t i = 0; i < 37; i++){
        try{
            f.add(i);
        }
        catch(...){
        
        }
    }
    
    // Watch the fireworks. Or maybe it's just a whimper.
    f.dump();
}