C++ template: add const to type if template arg is const

up vote
6
down vote

favorite

I have a class:

struct Foo 
 vector<float> data;
;

and I have a templated function that takes a Foo&:

template<typename T>
void f(T& arg) 
 using ftype = float *; // <-- would like this to be const float * if T is const
 ftype ptr = &arg.data[0];
 // ... do stuff with ptr ...

How can I make ptr be const float * iff T is const? I know about add_const and is_const but don't see how to use them here. (My real struct is more complicated and I don't have direct access to its internals; it's actually an OpenCV cv::Mat.)
I can use recent (C++14/C++17) features if needed.

I'll use it like this:

Foo foo1 = Foo();
f(foo1); // modifiable in f
const Foo cfoo = Foo();
f(cfoo); // const, should not be modifiable in f

asked 3 hours ago

GaryO

1,315915

4

If you wrote auto ptr = &arg.data[0];, decltype(ptr) would be const float*, no type traits needed
â€“Â Justin
2 hours ago

add a commentÂ |Â

up vote
6
down vote

favorite

I have a class:

struct Foo 
 vector<float> data;
;

and I have a templated function that takes a Foo&:

template<typename T>
void f(T& arg) 
 using ftype = float *; // <-- would like this to be const float * if T is const
 ftype ptr = &arg.data[0];
 // ... do stuff with ptr ...

I'll use it like this:

Foo foo1 = Foo();
f(foo1); // modifiable in f
const Foo cfoo = Foo();
f(cfoo); // const, should not be modifiable in f

asked 3 hours ago

GaryO

1,315915

4

If you wrote auto ptr = &arg.data[0];, decltype(ptr) would be const float*, no type traits needed
â€“Â Justin
2 hours ago

add a commentÂ |Â

up vote
6
down vote

favorite

I have a class:

struct Foo 
 vector<float> data;
;

and I have a templated function that takes a Foo&:

template<typename T>
void f(T& arg) 
 using ftype = float *; // <-- would like this to be const float * if T is const
 ftype ptr = &arg.data[0];
 // ... do stuff with ptr ...

I'll use it like this:

Foo foo1 = Foo();
f(foo1); // modifiable in f
const Foo cfoo = Foo();
f(cfoo); // const, should not be modifiable in f

asked 3 hours ago

GaryO

1,315915

I have a class:

struct Foo 
 vector<float> data;
;

and I have a templated function that takes a Foo&:

template<typename T>
void f(T& arg) 
 using ftype = float *; // <-- would like this to be const float * if T is const
 ftype ptr = &arg.data[0];
 // ... do stuff with ptr ...

I'll use it like this:

Foo foo1 = Foo();
f(foo1); // modifiable in f
const Foo cfoo = Foo();
f(cfoo); // const, should not be modifiable in f

c++ c++11 templates

asked 3 hours ago

GaryO

1,315915

asked 3 hours ago

GaryO

1,315915

asked 3 hours ago

GaryO

1,315915

asked 3 hours ago

GaryO

1,315915

asked 3 hours ago

GaryO

1,315915

4

If you wrote auto ptr = &arg.data[0];, decltype(ptr) would be const float*, no type traits needed
â€“Â Justin
2 hours ago

add a commentÂ |Â

4

If you wrote auto ptr = &arg.data[0];, decltype(ptr) would be const float*, no type traits needed
â€“Â Justin
2 hours ago

If you wrote auto ptr = &arg.data[0];, decltype(ptr) would be const float*, no type traits needed
â€“Â Justin
2 hours ago

add a commentÂ |Â

2 Answers
2

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up vote
7
down vote

accepted

Most likely, you actually just want the type of the expression &arg.data[0], for which you can use decltype.

You can also use std::conditional to distinguish the cases.

template<typename T>
void f(T& arg) 
 // C++17: using ftype = std::conditional_t<std::is_const_v<T>, const float *, float *>;
 using ftype = typename std::conditional<std::is_const<T>::value, const float *, float *>::type;

 ftype ptr = &arg.data[0];
 // ... do stuff with ptr ...

If instead of float *, you had a second type parameter U, you would include std::add_const

template<typename T, typename U = float *>
void f(T& arg) 
 // C++17: using ftype = std::conditional_t<std::is_const_v<T>, std::add_const_t<U>, U>;
 using ftype = typename std::conditional<std::is_const<T>::value, typename std::add_const<U>::type, U>::type;
 // ... do stuff with ftype ...

I have marked where C++14 and C++17 has nicer syntax for equivalent use. C++11 lack of template usings and template variables results in verbose type functions :(.

edited 11 mins ago

answered 2 hours ago

Caleth

12.8k11431

2

The big win is for C++14: _t versus typename ::value. C++17 is only ::value vs _v.
â€“Â Jarod42
2 hours ago

add a commentÂ |Â

up vote
2
down vote

template<class Src, class Dst>
using transcribe_const_t = std::conditional_t<std::is_const<Src>, Dst const, Dst>;
template<class Src, class Dst>
using transcribe_volatile_t = std::conditional_t<std::is_volatile<Src>, Dst volatile, Dst>;
template<class Src, class Dst>
using transcribe_cv_t = transcribe_const_t< Src, transcribe_volatile_t< Src, Dst> >;

now

template<typename T>
void f(T& arg) 
 using ftype = transcribe_cv_t<T, float>*;
 ftype ptr = &arg.data[0];
 // ... do stuff with ptr ...

does what you ask (and a bit more).

edited 15 mins ago

Jarod42

108k1297172

answered 1 hour ago

Yakk - Adam Nevraumont

171k18175353

add a commentÂ |Â

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2 Answers
2

active

oldest

votes

2 Answers
2

active

oldest

votes

up vote
7
down vote

accepted

Most likely, you actually just want the type of the expression &arg.data[0], for which you can use decltype.

You can also use std::conditional to distinguish the cases.

template<typename T>
void f(T& arg) 
 // C++17: using ftype = std::conditional_t<std::is_const_v<T>, const float *, float *>;
 using ftype = typename std::conditional<std::is_const<T>::value, const float *, float *>::type;

 ftype ptr = &arg.data[0];
 // ... do stuff with ptr ...

If instead of float *, you had a second type parameter U, you would include std::add_const

template<typename T, typename U = float *>
void f(T& arg) 
 // C++17: using ftype = std::conditional_t<std::is_const_v<T>, std::add_const_t<U>, U>;
 using ftype = typename std::conditional<std::is_const<T>::value, typename std::add_const<U>::type, U>::type;
 // ... do stuff with ftype ...

I have marked where C++14 and C++17 has nicer syntax for equivalent use. C++11 lack of template usings and template variables results in verbose type functions :(.

edited 11 mins ago

answered 2 hours ago

Caleth

12.8k11431

2

The big win is for C++14: _t versus typename ::value. C++17 is only ::value vs _v.
â€“Â Jarod42
2 hours ago

add a commentÂ |Â

up vote
7
down vote

accepted

Most likely, you actually just want the type of the expression &arg.data[0], for which you can use decltype.

You can also use std::conditional to distinguish the cases.

template<typename T>
void f(T& arg) 
 // C++17: using ftype = std::conditional_t<std::is_const_v<T>, const float *, float *>;
 using ftype = typename std::conditional<std::is_const<T>::value, const float *, float *>::type;

 ftype ptr = &arg.data[0];
 // ... do stuff with ptr ...

If instead of float *, you had a second type parameter U, you would include std::add_const

template<typename T, typename U = float *>
void f(T& arg) 
 // C++17: using ftype = std::conditional_t<std::is_const_v<T>, std::add_const_t<U>, U>;
 using ftype = typename std::conditional<std::is_const<T>::value, typename std::add_const<U>::type, U>::type;
 // ... do stuff with ftype ...

I have marked where C++14 and C++17 has nicer syntax for equivalent use. C++11 lack of template usings and template variables results in verbose type functions :(.

edited 11 mins ago

answered 2 hours ago

Caleth

12.8k11431

2

The big win is for C++14: _t versus typename ::value. C++17 is only ::value vs _v.
â€“Â Jarod42
2 hours ago

add a commentÂ |Â

up vote
7
down vote

accepted

Most likely, you actually just want the type of the expression &arg.data[0], for which you can use decltype.

You can also use std::conditional to distinguish the cases.

template<typename T>
void f(T& arg) 
 // C++17: using ftype = std::conditional_t<std::is_const_v<T>, const float *, float *>;
 using ftype = typename std::conditional<std::is_const<T>::value, const float *, float *>::type;

 ftype ptr = &arg.data[0];
 // ... do stuff with ptr ...

If instead of float *, you had a second type parameter U, you would include std::add_const

template<typename T, typename U = float *>
void f(T& arg) 
 // C++17: using ftype = std::conditional_t<std::is_const_v<T>, std::add_const_t<U>, U>;
 using ftype = typename std::conditional<std::is_const<T>::value, typename std::add_const<U>::type, U>::type;
 // ... do stuff with ftype ...

I have marked where C++14 and C++17 has nicer syntax for equivalent use. C++11 lack of template usings and template variables results in verbose type functions :(.

edited 11 mins ago

answered 2 hours ago

Caleth

12.8k11431

Most likely, you actually just want the type of the expression &arg.data[0], for which you can use decltype.

You can also use std::conditional to distinguish the cases.

template<typename T>
void f(T& arg) 
 // C++17: using ftype = std::conditional_t<std::is_const_v<T>, const float *, float *>;
 using ftype = typename std::conditional<std::is_const<T>::value, const float *, float *>::type;

 ftype ptr = &arg.data[0];
 // ... do stuff with ptr ...

If instead of float *, you had a second type parameter U, you would include std::add_const

template<typename T, typename U = float *>
void f(T& arg) 
 // C++17: using ftype = std::conditional_t<std::is_const_v<T>, std::add_const_t<U>, U>;
 using ftype = typename std::conditional<std::is_const<T>::value, typename std::add_const<U>::type, U>::type;
 // ... do stuff with ftype ...

I have marked where C++14 and C++17 has nicer syntax for equivalent use. C++11 lack of template usings and template variables results in verbose type functions :(.

edited 11 mins ago

answered 2 hours ago

Caleth

12.8k11431

edited 11 mins ago

answered 2 hours ago

Caleth

12.8k11431

answered 2 hours ago

Caleth

12.8k11431

answered 2 hours ago

Caleth

12.8k11431

2

The big win is for C++14: _t versus typename ::value. C++17 is only ::value vs _v.
â€“Â Jarod42
2 hours ago

add a commentÂ |Â

2

The big win is for C++14: _t versus typename ::value. C++17 is only ::value vs _v.
â€“Â Jarod42
2 hours ago

The big win is for C++14: _t versus typename ::value. C++17 is only ::value vs _v.
â€“Â Jarod42
2 hours ago

add a commentÂ |Â

up vote
2
down vote

template<class Src, class Dst>
using transcribe_const_t = std::conditional_t<std::is_const<Src>, Dst const, Dst>;
template<class Src, class Dst>
using transcribe_volatile_t = std::conditional_t<std::is_volatile<Src>, Dst volatile, Dst>;
template<class Src, class Dst>
using transcribe_cv_t = transcribe_const_t< Src, transcribe_volatile_t< Src, Dst> >;

now

template<typename T>
void f(T& arg) 
 using ftype = transcribe_cv_t<T, float>*;
 ftype ptr = &arg.data[0];
 // ... do stuff with ptr ...

does what you ask (and a bit more).

edited 15 mins ago

Jarod42

108k1297172

answered 1 hour ago

Yakk - Adam Nevraumont

171k18175353

add a commentÂ |Â

up vote
2
down vote

template<class Src, class Dst>
using transcribe_const_t = std::conditional_t<std::is_const<Src>, Dst const, Dst>;
template<class Src, class Dst>
using transcribe_volatile_t = std::conditional_t<std::is_volatile<Src>, Dst volatile, Dst>;
template<class Src, class Dst>
using transcribe_cv_t = transcribe_const_t< Src, transcribe_volatile_t< Src, Dst> >;

now

template<typename T>
void f(T& arg) 
 using ftype = transcribe_cv_t<T, float>*;
 ftype ptr = &arg.data[0];
 // ... do stuff with ptr ...

does what you ask (and a bit more).

edited 15 mins ago

Jarod42

108k1297172

answered 1 hour ago

Yakk - Adam Nevraumont

171k18175353

add a commentÂ |Â

up vote
2
down vote

template<class Src, class Dst>
using transcribe_const_t = std::conditional_t<std::is_const<Src>, Dst const, Dst>;
template<class Src, class Dst>
using transcribe_volatile_t = std::conditional_t<std::is_volatile<Src>, Dst volatile, Dst>;
template<class Src, class Dst>
using transcribe_cv_t = transcribe_const_t< Src, transcribe_volatile_t< Src, Dst> >;

now

template<typename T>
void f(T& arg) 
 using ftype = transcribe_cv_t<T, float>*;
 ftype ptr = &arg.data[0];
 // ... do stuff with ptr ...

does what you ask (and a bit more).

edited 15 mins ago

Jarod42

108k1297172

answered 1 hour ago

Yakk - Adam Nevraumont

171k18175353

template<class Src, class Dst>
using transcribe_const_t = std::conditional_t<std::is_const<Src>, Dst const, Dst>;
template<class Src, class Dst>
using transcribe_volatile_t = std::conditional_t<std::is_volatile<Src>, Dst volatile, Dst>;
template<class Src, class Dst>
using transcribe_cv_t = transcribe_const_t< Src, transcribe_volatile_t< Src, Dst> >;

now

template<typename T>
void f(T& arg) 
 using ftype = transcribe_cv_t<T, float>*;
 ftype ptr = &arg.data[0];
 // ... do stuff with ptr ...

does what you ask (and a bit more).

edited 15 mins ago

Jarod42

108k1297172

answered 1 hour ago

Yakk - Adam Nevraumont

171k18175353

edited 15 mins ago

Jarod42

108k1297172

edited 15 mins ago

Jarod42

108k1297172

edited 15 mins ago

Jarod42

108k1297172

answered 1 hour ago

Yakk - Adam Nevraumont

171k18175353

answered 1 hour ago

Yakk - Adam Nevraumont

171k18175353

answered 1 hour ago

Yakk - Adam Nevraumont

171k18175353

add a commentÂ |Â

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