如何创建并正确访问指向std::vector的指针构成的std::vector？

Hey there, let's break down why your code is failing and how to fix it right away!

The Root Problem #

Your vecP is a std::vector<std::vector<size_t>*>—meaning each element inside it is a pointer to a std::vector<size_t>, not an actual vector object. When you try to do vecP[1].resize(6) or vecP[1][3] =7, you're using the . operator (for object members) on a pointer, which is invalid in C++.

Correct Ways to Access the Vector #

You have two straightforward options to fix this:

Option 1: Use the -> Operator (Pointer Member Access)

Since vecP[1] is a pointer, use -> to call member functions or access elements:

// Resize the second vector (index 1) to 6 elements
vecP[1]->resize(6);

// Set the 4th element (index 3) to 7 (with bounds checking)
vecP[1]->at(3) = 7; 
// Or use operator[] directly (no bounds checking):
// vecP[1]->operator[](3) = 7;

Option 2: Dereference the Pointer First

You can use * to convert the pointer into a reference to the actual vector object, then use the . operator as usual:

// Dereference the pointer, then call resize
(*vecP[1]).resize(6);

// Dereference and access the element
(*vecP[1])[3] = 7;

Note: The parentheses around *vecP[1] are critical here—operator precedence means vecP[1][3] would try to index the pointer itself (which is wrong), so we need to dereference first.

Bonus: A Safer Alternative (Avoid Pointers If Possible) #

If you don't need dynamic memory allocation for these vectors, consider using a std::vector<std::vector<size_t>> instead. This eliminates pointer-related headaches and reduces risks like dangling pointers or memory leaks:

std::vector<size_t> vec1, vec2, vec3;
std::vector<std::vector<size_t>> vecP = {vec1, vec2, vec3};

// Now you can use . directly:
vecP[1].resize(6);
vecP[1][3] = 7;

If you need to modify the original vectors through vecP (instead of working with copies), use a vector of references via std::reference_wrapper:

#include <functional> // Required for std::reference_wrapper

std::vector<size_t> vec1, vec2, vec3;
std::vector<std::reference_wrapper<std::vector<size_t>>> vecP = {vec1, vec2, vec3};

// Access and modify the original vectors:
vecP[1].get().resize(6);
vecP[1].get()[3] = 7;

内容的提问来源于stack exchange，提问作者Matthew