String to double

Probably, the C++ standard library is unlimited. There are so many functions I've never heard about. For example, only today I found out that there is strtod function:

C: strtod in stdlib.h
C++: strtod in cstdlib

I knew about atof.

But this function does not work with the strings like "20nn". So, for example, how to check if this conatins a valid double?

I made even a function and tested it in this code:

#include <cstdio>
#include <string>

bool is_valid_double(const char* str)
{
    while (str && *str && *str == ' ')
        str++;

    if (*str && *str == '-')
        str++;

    while (str && *str)
    {
        if (!isdigit(*str) && (*str != '.'))
        {
            if (*str == ' ')
            {
                while (str && *str && *str == ' ')
                    str++;
                if (!*str)
                    return true;
            }
            return false;
        }
        str++;
    }

    return true;
}

int main()
{
    char sz[16] = { 0 };
    sprintf(sz, "%f", 20.0);
    bool b = is_valid_double(sz);

    strcpy(sz, "  20.00");
    b = is_valid_double(sz);

    strcpy(sz, "  20.00  ");
    b = is_valid_double(sz);

    strcpy(sz, "  -20.00  ");
    b = is_valid_double(sz);

    strcpy(sz, "  --20.00");
    b = is_valid_double(sz);

    strcpy(sz, "  20.00  2");
    b = is_valid_double(sz);

    strcpy(sz, "  20nn  ");
    b = is_valid_double(sz);

    return 0;
}

And then I see (thanks to EE), that there are a set of functions such as strtod, strtof, etc. Actually, I knew about strtol, so I could guess that should be also strtod. But I didn't. :)