How to create a Variant class similar to QVariant

I am trying to create a Variant class similar to Qt's QVariant. I've considered std::variant and std::any but unfortunately I need something analogous to the QVariant::value() function. But, I can't find a way to implement the function without getting errors like this:

note: candidate: template<class T> T Variant::value() const
inline T value() const {
^~~~~
note: template argument deduction/substitution failed:
note: couldn't deduce template parameter ‘T’


This answer seems to be what I want but I am trying to understand why that implementation works without receiving the "deduce template parameter 'T'" error on the return type. Any help would be greatly appreciated. Thank you.

Here's my .H:

class Variant

private:

enum VariantType
CHAR = 0,
UCHAR,
SCHAR,
SHORT,
USHORT,
INT,
UINT,
LONG,
ULONG,
LONGLONG,
ULONGLONG,
BOOL,
FLOAT,
DOUBLE,
LONGDOUBLE
;

public:

explicit Variant(int i);
explicit Variant(double d);

int ToInt() const;
double ToDouble() const;

private:

struct Internal

Internal(VariantType typeIn) :
type(typeIn)


union
char c;
unsigned char uc;
short s;
signed char sc;
unsigned short us;
int i;
unsigned int ui;
long l;
unsigned long ul;
long long ll;
unsigned long long ull;
bool b;
float f;
double d;
long double ld;
void *ptr;
value;
VariantType type;
;

Internal _data;

public:

template<typename T>
inline T value() const
// try to get int/double working first
if (_data.type == VariantType::DOUBLE)
return _data.value.d;
else if (_data.type == VariantType::INT)
return _data.value.i;


;


And here's my .CPP:

Variant::Variant(int i) : _data(INT)
_data.value.i = i;


Variant::Variant(double d) : _data(DOUBLE)
_data.value.d = d;


int Variant::ToInt() const
return _data.value.i;


double Variant::ToDouble() const
return _data.value.d;



EDIT.

std::variant

std::variant

1 Answer
1

The error comes from your call site, not the class definition. And the error tells you that Variant::value<T>() needs a <T>, but you did not provide any type argument with which to instantiate value<T>.

Variant::value<T>()

<T>

value<T>

This same restriction applies to QtVariant, it's not unique to your class.

QtVariant

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