my_setfield.cpp

#include "mex.h"

void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
{
    // check number of arguments
    if (nrhs != 3) {
        mexErrMsgIdAndTxt("MATLAB:minrhs", "Not enough input arguments.");
    }
    if (nlhs > 1) {
        mexErrMsgIdAndTxt("MATLAB:maxlhs", "Too many output arguments.");
    }
    if (nlhs == 0) {
        mexWarnMsgIdAndTxt("MATLAB:MustHaveOutput", "my_setfield must be called with an output.");
    }

    // check type of input arguments
    if (!mxIsStruct(prhs[0]) || mxGetNumberOfElements(prhs[0])!=1) {
        mexErrMsgIdAndTxt("MATLAB:mustBeStruct", "Not a scalar structure.");
    }
    if (!mxIsChar(prhs[1]) || mxGetM(prhs[1])!=1) {
        mexErrMsgIdAndTxt("MATLAB:mustBeFieldName", "Not a valid field name.");
    }

    // create output structure
    plhs[0] = mxDuplicateArray(prhs[0]);

    // name of field
    char *fieldname = mxArrayToString(prhs[1]);
    if (fieldname == NULL) {
        mexErrMsgIdAndTxt("MATLAB:NULL", "NULL pointer.");
    }

    // get corresponding field number
    int fieldnum = mxGetFieldNumber(plhs[0], fieldname);
    if (fieldnum == -1) {
        // add new field
        fieldnum = mxAddField(plhs[0], fieldname);
    } else {
        // free existing field value
        mxArray *pm = mxGetFieldByNumber(plhs[0], 0, fieldnum);
        if (pm != NULL) {
            mxDestroyArray(pm);
        }
    }
    mxFree(fieldname);

    // set field value
    if (fieldnum == -1) {
        mexErrMsgIdAndTxt("MATLAB:error", "Invalid field number.");
    }
    mxSetFieldByNumber(plhs[0], 0, fieldnum, mxDuplicateArray(prhs[2]));
}