Re: A malloc error in C++ - incorrect checksum for freed object

Jerry Coffin <>
Sun, 2 Sep 2007 17:47:13 -0600
In article <>,

[ ... ]

I don't like the idea of having a vector full of identical values just
for using it with inner_product. I _like_ the idea of using std
algorithms, though.

There is another way: make something that acts like a vector full of
identical values:

// warning: untested code
template <typename T>
class single_value {
    T v_;
    size_t size_;
    single_value(T v, size_t size) : v_(v), size_(size) {}

    double operator[](size_t index) {
        return v_;

    friend iterator;

    class iterator {
        size_t pos_;
        single_value v_;
        iterator(single_value const &v, size_t pos = 0)
            : v_(v), pos_(pos)

        iterator &operator++() {
            return *this;

        double operator*() { return v_.v_; }
        bool operator==(iterator const &other) {
            return pos_ == other.pos_;

    iterator begin() { return iterator(this); }
    iterator end() { return iterator(this, size_); }

Another possibility would be to use valarrays. They were really intended
for this kind of computational work, and can often handle it quite

// Warning: only minimally tested
template <class T>
T const variance(T *values, size_t size) {
    std::valarray<T> const v(values, size);

    T average = v.sum() / v.size();

    std::valarray<T> diffs = v-average;

    diffs *= diffs;

    return diffs.sum()/diffs.size();

As this is written at the moment, it accepts a pointer and size, but it
would be fairly easy to allow it to accept input as a pair of iterators
or such. Given the OP's problem, using valarrays throughout might be a
possibility as well, and I didn't want to spend a lot of code on
something that might easily not matter.

Whether this is better or worse is open to some question -- on one hand,
it does temporarily store all the (squared) deviations from the mean, so
it uses extra memory that isn't strictly necessary. OTOH, I think it's
much easier to read and understand than either the original code or your
code. That's likely good in educational code, but not so good for a
library that will be used heavily but only rarely read. OTOH, for big
vector-based machines (e.g. Crays) most valarray operations are easy to
run on the vector processer, in which case they should be quite fast.


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