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:

• std::string
• char const * (read-only: only as a bound parameter for a query)
• short, int, long long
• long: since the size of a long is 64bit on UNIX/Linux 64bit systems and 32bit otherwise, it is mapped to an integer or a bigint depending on the environment.
• float, double
• enum types
• bool
• std::vector<unsigned char> (binary data)
• boost::optional<T>: to make the type optional (allowing an SQL null value)
• std::optional<T>: to make the type optional (allowing an SQL null value, C++17 only)
• std::chrono::system_clock::time_point: time stamp
• std::chrono::duration<int, std::milli>: time interval

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

• WDate
• WDateTime
• WTime
• WString
• Json::Object
• Json::Array

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

enum class Pet {
  Cat,
  Dog,
  Other
};
 
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)
  {
    return sql_value_traits<std::string>::type(conn, 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.