CODEWITHSUNDEEP
c++shared-ptr
WhatABeautifulWorld
WhatABeautifulWorld

Reputation: 3388

const in shared_ptr: do I need two or only one

I run into this piece of code:

const std::shared_ptr<const T>& a_shared_pointer,

I am really curious what do the two const mean? Do they mean the same thing? a_shared_pointer is a shared pointer that points to something we can't modify?

Upvotes: 0

Views: 271

Answers (2)

Richard Hodges
Richard Hodges

Reputation: 69864

A shared pointer is analgous to a raw pointer in terms of dereferencing and constness.

Example:

Note that below, the term [const] means we can choose whether to insert const or not:

[const] X * [const] p;

Gives us 4 options with regards to constness:

X* p; - p is a mutable pointer (it can be changed to point at a different X) to a mutable X

const X *p - p is a mutable pointer to an immutable X

X * const p; - p is an immutable pointer (can only point to this X) to a mutable X

const X * const p; - p is an immutable pointer (can only point to this X) to an immutable X

Similarly with shared_ptr:

std::shared_ptr<T> &p - reference to a mutable pointer to a mutable T

const std::shared_ptr<T> &p - reference to an immutable pointer to a mutable T

std::shared_ptr<const T> &p - reference to a mutable pointer to an immutable T

const std::shared_ptr<const T> &p - reference to an immutable pointer to an immutable T

Upvotes: 3

paler123
paler123

Reputation: 976

a_shared pointer is a reference to a const std::shared_ptr (e.g. you can't call not const operations on the pointer through this reference, like reset etc.), pointing at a const T meaning that the pointed to object is also const (so again, you can't do a_shared_ptr->non_const_method()).

Upvotes: 1

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