Re: Returning a template depending on arguments

Richard Damon <>
Sun, 28 Aug 2011 18:43:40 -0400
On 8/28/11 10:38 AM, daniele lugli wrote:

Dear all,
this class:

template<int m, int n>
class SmallMatrix
    typedef double myData [m] [n];
    myData M;

    SmallMatrix () {
        for (int i = 0; i< m; i++) {
            for (int j = 0; j< n; j++) {
                M[i][j] = 0.;
    virtual ~SmallMatrix () {}
    operator myData& () {
        return M;
         operator const myData& () const {
        return M;

compiles fine with g++-4.4.3 and does what I expect it to do, eg I can
declare a SmallMatrix<3,4> m; and then access its elements as m[i][j].

Then I tried to add an operator* like this:

    SmallMatrix<m, n1> operator* (const SmallMatrix<n, n1> & f) const {
        SmallMatrix<m, n1> result;
        for (int i = 0; i< m; i++) {
            for (int j = 0; j< n1; j++) {
                result[i][j] = 0.;
                for (int k = 0; k< n; k++) {
                    result[i][j] += M[i][k] * f[k][j];
        return result;

what I would like to do is:

SmallMatrix<2, 3> m23;
SmallMatrix<3, 4> m34;
SmallMatrix<2, 4> m24;
m24 = m23 * m34;

but the class SmallMatrix with the operator* method in it does not
compile; I get:
  'n1' was not declared in this scope
template argument 2 is invalid

clearly because n1 is not known before it appears in the return type
declaration; it becomes known only after the argument of operator* is

I cannot see any way of getting what I would like to have. Is it
possible or it is one of the things one cannot do with c++?


I think you want operator* to be declared as

template<int n1> SmallMatrix<m, n1> operator*(const SmallMatrix<n,n1>&
f) const {

as you are introducing another template parameter into the code, so need
to have nested templates.

The more canonical method would be to make operator * a free function
(maybe a friend) declared outside the class as:

template<int m, int n, int n1> SmallMatrix<m, n1> operator*(const
SmallMatrix<m,n>& f, const SmallMatrix<n,n1>& g){

so that if there is a class convertible to SmallMatrix it can use the

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