This function is intended to remove leading and trailing whitespace from a string. How can it be made more efficent, for example can the two for loops be combined into one?

string trim(string str)
{
    int i = 0;
    for (char c : str)
    {
        if (!isspace(c))
            break;
        i++;
    }

    string trimmed = str.substr(i, (str.length()-i));

    i = 0;
    for (char c : str)
    {
        if (isspace(c))
            break;
        i++;
    }

    trimmed = trimmed.substr(0, i);
    return trimmed;
}

This function is intended to remove leading and trailing whitespace from a string. How can it be made more efficient? For example, can the two for loops be combined into one?

string trim(string str)
{
    int i = 0;
    for (char c : str)
    {
        if (!isspace(c))
            break;
        i++;
    }

    string trimmed = str.substr(i, (str.length()-i));

    i = 0;
    for (char c : str)
    {
        if (isspace(c))
            break;
        i++;
    }

    trimmed = trimmed.substr(0, i);
    return trimmed;
}