How do you restrict `resize()` from being called after constructing a vector?

user_120888 0
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I'm building a class that exposes a sequential container, with a fixed length, but the length isn't known at compile-time.

So when an instance of my class is constructed, a parameter is passed in, to indicate how big the vector needs to be.

But the length needs to be fixed after construction.
I need to guarantee that the resize() function cannot be invoked, while still allowing other parts of my code to modify individual elements within the vector.
(In other words, I can't simply expose the vector as vector<T> const&)

The same goes for any other function which modifies the length, such as insert(), push_back(), etc.
These functions need to be restricted or hidden.

Is this possible?
Or do I really need to build my own fixed_vector wrapper class to hide the undesired functions?

vector containers collections c++
1 Answers

Since C++20 you can return a std::span to the range in the vector. This allows access to the size and modifiable access to the elements, but not the vector's modifiers.

For example:

#include<vector>
#include<span>

class A {
    std::vector<int> vec;
public:
    /*...*/
    auto getVec() {
        return std::span(vec);
    }
 };

The return value can be used as a range, but there is no access to the container interface.

Depending on the types and initialization required, you may also be able to use an array std::unique_ptr instead of a std::vector if you know the size won't change. However that doesn't store the size, which you would then need to store yourself:

#include<vector>
#include<span>

class A {
    std::size_t vec_size;
    std::unique_ptr<int[]> vec;
public:
    A(std::size_t size) : vec_size(size), vec(std::make_unique<int[]>(size)) { }

    auto getVec() {
        return std::span(vec, vec_size);
    }
 };

This may be slightly more space efficient since it doesn't require accounting for a difference in vector size and capacity.

user_177325220
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