A typedef is aliasing a type.
Syntax: typedef type name
Example: typedef int length
Uses:
1. Give meaningful shorter names for really long type names. Specially, in case of template types and function pointers.
2. Change the underlying type when needed.
Requirement: I am writing a math library with support for unsigned integers of 8 bit long.
without Typedef,
Why only 8 bit long!... The same library can handle 16 bit integers.
Uh-Oh! I must change all the types throughout the library.
with Typedef's,
voila, I can now support 16 bit integers with a single change!!
3. Document Types.
The typedef's are weakly typed and cause problems. During syntax and semantic analysis (C++ is not context free grammar), all the alias types are resolved to the same underlying type.
1. No type safety: The synonym types can be used interchangeably or together,
2. Prevent overloading: Both aliases are actually the same types.
Boost library provides a wrapper to achieve strong type'ing with typedef's with a macro BOOST_STRONG_TYPEDEF.
Epilogue:
There are reasons why no proposal's for strong typedef (N1706 and N1891) made it to the standard. Could be simply because it can be achieved with a wrapper. Not delving deeper into this!
Syntax: typedef type name
Example: typedef int length
Uses:
1. Give meaningful shorter names for really long type names. Specially, in case of template types and function pointers.
typedef myLongLongMeaningfulType tMeaningfulType typedef std::map<int,std::map<int, std::vector<int> > > doubleIndexMap;
2. Change the underlying type when needed.
Requirement: I am writing a math library with support for unsigned integers of 8 bit long.
without Typedef,
uint8_t add (uint8_t a, uint8_t b);
Why only 8 bit long!... The same library can handle 16 bit integers.
Uh-Oh! I must change all the types throughout the library.
with Typedef's,
typedef uint8_t supportType_t; supportType_t add (supportType_t a, supportType_t b);
typedef uint16_t supportType_t;
3. Document Types.
typedef uint8_t length_t; // always stores lengths in uint8 format. typedef uint8_t area_t; // always stores area's in uint8 format.
The typedef's are weakly typed and cause problems. During syntax and semantic analysis (C++ is not context free grammar), all the alias types are resolved to the same underlying type.
typedef uint8_t length_t; typedef uint8_t area_t;
length_t
and
area_t
are still uint8.
1. No type safety: The synonym types can be used interchangeably or together,
length_t m_boxLength = 10_CM;
area_t m_boxArea = 100_SQCM;
length_t getLength()
{
return m_boxArea; // Bug, no compiler warning. both area and length are uint8
}
if(box1.getLength() == box2.getArea()) // Bug, no compiler warning. both area and length are uint8
2. Prevent overloading: Both aliases are actually the same types.
Boost library provides a wrapper to achieve strong type'ing with typedef's with a macro BOOST_STRONG_TYPEDEF.
Epilogue:
There are reasons why no proposal's for strong typedef (N1706 and N1891) made it to the standard. Could be simply because it can be achieved with a wrapper. Not delving deeper into this!
