I have a method which goes through a vector of type StateItem

for(int i = 0; i < stateItems.size(); i++)
    StateItem &st = stateItems[i];
    switch (st.state)
        case Effect:
            result += std::string(", \"effect\": ") + st.value.value;
            break;
        case Hue:
            result += std::string(", \"hue\": ") + st.value.intValue.str();
            break;
        case On:
            result += std::string(", \"on\": ") + std::string(st.value.value);
            break;
        default:
            break;
In the case Hue I get the following Compiler error:
Member reference base type 'int' is not a structure or union
I can´t understand the problem here.

Can anyone of you please help me?
You're trying to call a member function on intValue, which has type int. int isn't a class type, so has no member functions.
In C++11 or later, there's a handy std::to_string function to convert int and other built-in types to std::string:
result += ", \"hue\": " + std::to_string(st.value.intValue);
Historically, you'd have to mess around with string streams:
    std::stringstream ss;
    ss << st.value.intValue;
    result += ", \"hue\": " + ss.str();
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C++ – What does the C++ standard state the size of int, long type to be

The C++ standard does not specify the size of integral types in bytes, but it specifies minimum ranges they must be able to hold. You can infer minimum size in bits from the required range. You can infer minimum size in bytes from that and the value of the CHAR_BIT macro that defines the number of bits in a byte. In all but the most obscure platforms it's 8, and it can't be less than 8.
One additional constraint for char is that its size is always 1 byte, or CHAR_BIT bits (hence the name). This is stated explicitly in the standard.
The C standard is a normative reference for the C++ standard, so even though it doesn't state these requirements explicitly, C++ requires the minimum ranges required by the C standard (page 22), which are the same as those from Data Type Ranges on MSDN:
signed char: -127 to 127 (note, not -128 to 127; this accommodates 1's-complement and sign-and-magnitude platforms)
unsigned char: 0 to 255
"plain" char: same range as signed char or unsigned char, implementation-defined
signed short: -32767 to 32767
unsigned short: 0 to 65535
signed int: -32767 to 32767
unsigned int: 0 to 65535
signed long: -2147483647 to 2147483647
unsigned long: 0 to 4294967295
signed long long: -9223372036854775807 to 9223372036854775807
unsigned long long: 0 to 18446744073709551615
A C++ (or C) implementation can define the size of a type in bytes sizeof(type) to any value, as long as
the expression sizeof(type) * CHAR_BIT evaluates to a number of bits high enough to contain required ranges, and
the ordering of type is still valid (e.g. sizeof(int) <= sizeof(long)).
Putting this all together, we are guaranteed that:
char, signed char, and unsigned char are at least 8 bits
signed short, unsigned short, signed int, and unsigned int are at least 16 bits
signed long and unsigned long are at least 32 bits
signed long long and unsigned long long are at least 64 bits
No guarantee is made about the size of float or double except that double provides at least as much precision as float.
The actual implementation-specific ranges can be found in <limits.h> header in C, or <climits> in C++ (or even better, templated std::numeric_limits in <limits> header).
For example, this is how you will find maximum range for int:
#include <limits.h>
const int min_int = INT_MIN;
const int max_int = INT_MAX;
C++:
#include <limits>
const int min_int = std::numeric_limits<int>::min();
const int max_int = std::numeric_limits<int>::max();
    
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The problem is with the operator precedence. Instead of:
*i->update( this );
(*i)->update( this );
Otherwise, it will be interpreted as *(i->update( this )), which tries to call a method on a pointer, resulting in the error message.