Read expression from Mathematica via Mathlink C++

Question

This is a simple C++ program to send an expression to the_Mathematica_ Kernel and get its response.

#include <mathlink.h> //mathlink header
#include <stdio.h> //standard io header
#include <stdlib.h>
#include <iostream>
using namespace std;

/* initialising variables */
MLENV env = (MLENV)0;
MLINK link = (MLINK)0;

int MLEvaluate(MLINK mlp, char* s)
{
    return MLPutFunction(mlp, "EvaluatePacket", 1L)
        && MLPutFunction(mlp, "ToExpression", 1L)
        && MLPutString(mlp, s)
        && MLEndPacket(mlp);
} /* MLEvaluate */

int main(int argc, char *argv[]) {
    int errno;
    int packet;
    char input[255];

    /* initialises MathLink environemnt object */
    env = MLInitialize(0);

    /* links the program to the MathKernel */
    link = MLOpenString(env, "-linkmode launch -linkname 'C:\\Program Files\\Wolfram Research\\Mathematica\\9.0\\math.exe'", &errno);
    MLActivate(link); //activates a MathLink connection with a mathlink program

    cout << "Enter your expression: ";
    cin >> input;
    MLEvaluate(link, input);

    /* get packets until we find a ReturnPacket or error */
    while ((packet = MLNextPacket(link)) && packet != RETURNPKT) MLNewPacket(link);

    /* once the results is returned */
    if (MLError(link)) printf("\nError Has Occured!\n");  //if an error is returned    
    else {                                                //if an integer is returned
        int result;
        const char *string;
        MLGetString(link, &string);
        printf("\nResult: %s\n", string);

        MLReleaseString(link, string);
    }

    printf("\nPress Enter to Exit...");
    getchar(); //holds the console till users presses enter
    getchar(); //holds the console till users presses enter
    return 0;
}

When I type the expression show below in response to the console prompt

2+2

I get an answer. However, when I type in

2+a

I dont seem to be able to read the answer as string

Where did I go wrong? Please advise

Answer 1

Yes: Just use

return MLPutFunction(mlp, "EvaluatePacket", 1L)
    && MLPutFunction(mlp, "ToString", 1L)       // <<<--- !
    && MLPutFunction(mlp, "ToExpression", 1L)
    /* ... */