Isdigit Problem

I want to take the string tokens and test if they are integers. If not, an exception should be thrown. According to isdigit - C++ Reference , isdigit should return 0 if it is not a digit. But it is returning 0 for anything I input. Why is this? The code snippet is below.

Code:

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BinaryTree BinaryTree::setTree(char* str) {
    char* pStr = strtok(str, "()");
    BinaryTree result;
    while(pStr != NULL) {
        int token = atoi(pStr);
        if(isdigit(token) == 0) {
            std::cout<<pStr<<" is nTree"<<std::endl;
            std::cout<<token<<" is not an int."<<std::endl;
            throw InvalidInputException();
        }

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You're probably confused because isdigit()'s parameter is prototyped as int c. isdigit() returns true if c is in the range of '0' - '9', in other words, treat it like the parameter is a char.