Wt  4.5.0
Static Public Member Functions | List of all members
Wt::Dbo::sql_value_traits< V, Enable > Class Template Reference

Traits class for value types. More...

#include <Wt/Dbo/SqlTraits.h>

Static Public Member Functions

static const char * type (SqlConnection *connection, int size)
 Returns the SQL type name. More...
 
static void bind (const V &v, SqlStatement *statement, int index, int size)
 Binds a value to a statement parameter. More...
 
static bool read (V &v, SqlStatement *statement, int column, int size)
 Reads a result from an executed query. More...
 

Detailed Description

template<typename V, class Enable = void>
class Wt::Dbo::sql_value_traits< V, Enable >

Traits class for value types.

This traits class may be specialized for a custom type V, to add dbo support for custom types. A value type has a one-to-one mapping to a single database column.

The library has built-in support for:

In <Wt/Dbo/WtSqlTraits.h>, traits classes are also provided for:

Example for an enum that is saved as a string rather than an int:

enum class Pet {
Cat,
Dog,
};
std::string petToString(Pet p) {
switch (p) {
case Pet::Cat:
return "cat";
case Pet::Dog:
return "dog";
case Pet::Other:
return "other";
}
throw std::invalid_argument("Unknown pet type: " + std::to_string(static_cast<int>(p)));
}
Pet petFromString(const std::string &s) {
if (s == "cat")
return Pet::Cat;
else if (s == "dog")
return Pet::Dog;
else if (s == "other")
return Pet::Other;
else
throw std::invalid_argument("Unknown pet type: " + s);
}
namespace Wt {
namespace Dbo {
template<>
struct sql_value_traits<Pet>
{
static std::string type(SqlConnection *conn, int size)
{
}
static void bind(Pet p, SqlStatement *statement, int column, int size)
{
statement->bind(column, petToString(p));
}
static bool read(Pet &p, SqlStatement *statement, int column, int size)
{
std::string s;
bool result = statement->getResult(column, &s, size);
if (!result)
return false;
p = petFromString(s);
return true;
}
};
} // Dbo
} // Wt
See also
query_result_traits

Member Function Documentation

◆ bind()

template<typename V, class Enable = void>
static void Wt::Dbo::sql_value_traits< V, Enable >::bind ( const V &  v,
SqlStatement statement,
int  index,
int  size 
)
static

Binds a value to a statement parameter.

The value v must be bound to parameter with index index in the statement.

See also
SqlStatement::bind()

◆ read()

template<typename V, class Enable = void>
static bool Wt::Dbo::sql_value_traits< V, Enable >::read ( V &  v,
SqlStatement statement,
int  column,
int  size 
)
static

Reads a result from an executed query.

The value v must be read from result column column in the statement.

Returns true if the value was not null. This result may be used by the boost::optional<V> specialization to support fields that may have null values.

See also
SqlStatement::getResult()

◆ type()

template<typename V, class Enable = void>
static const char* Wt::Dbo::sql_value_traits< V, Enable >::type ( SqlConnection connection,
int  size 
)
static

Returns the SQL type name.

The size (for strings) is a hint and may be ignored by a back-end.

This will usually return a type ending with " not null" except for C++ types that support null values. For a normal c++ value type T, boost::optional<T> has been specialized to allow for null values.


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