float vertices[] = {
        0.5f,  0.5f, 0.0f,  // top right
        0.5f, -0.5f, 0.0f,  // bottom right
       -0.5f, -0.5f, 0.0f,  // bottom left
       -0.5f,  0.5f, 0.0f   // top left 
   };
   unsigned int indices[] = {  // note that we start from 0!
       0, 1, 3,  // first Triangle
       1, 2, 3   // second Triangle
   };

   Mesh* mesh = new Mesh(vertices, indices);

The Ctor:

Mesh::Mesh(float* vertices, unsigned int* indices)
{
    glGenVertexArrays(1, &VAO);
    glGenBuffers(1, &VBO);
    glGenBuffers(1, &EBO);
    // bind the Vertex Array Object first, then bind and set vertex buffer(s), and then configure vertex attributes(s).
    glBindVertexArray(VAO);

    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);

    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);

    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(float), (void*)0);
    glEnableVertexAttribArray(0);

    // note that this is allowed, the call to glVertexAttribPointer registered VBO as the vertex attribute's bound vertex buffer object so afterwards we can safely unbind
    glBindBuffer(GL_ARRAY_BUFFER, 0);

    // remember: do NOT unbind the EBO while a VAO is active as the bound element buffer object IS stored in the VAO; keep the EBO bound.
    //glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);

    // You can unbind the VAO afterwards so other VAO calls won't accidentally modify this VAO, but this rarely happens. Modifying other
    // VAOs requires a call to glBindVertexArray anyways so we generally don't unbind VAOs (nor VBOs) when it's not directly necessary.
    glBindVertexArray(0);
}

This does not work, but this does:

Mesh::Mesh()
{
    float vertices[] = {
        0.5f,  0.5f, 0.0f,  // top right
        0.5f, -0.5f, 0.0f,  // bottom right
       -0.5f, -0.5f, 0.0f,  // bottom left
       -0.5f,  0.5f, 0.0f   // top left 
   };
   unsigned int indices[] = {  // note that we start from 0!
       0, 1, 3,  // first Triangle
       1, 2, 3   // second Triangle
   };


    glGenVertexArrays(1, &VAO);
    glGenBuffers(1, &VBO);
    glGenBuffers(1, &EBO);
    // bind the Vertex Array Object first, then bind and set vertex buffer(s), and then configure vertex attributes(s).
    glBindVertexArray(VAO);

    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);

    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);

    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(float), (void*)0);
    glEnableVertexAttribArray(0);

    // note that this is allowed, the call to glVertexAttribPointer registered VBO as the vertex attribute's bound vertex buffer object so afterwards we can safely unbind
    glBindBuffer(GL_ARRAY_BUFFER, 0);

    // remember: do NOT unbind the EBO while a VAO is active as the bound element buffer object IS stored in the VAO; keep the EBO bound.
    //glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);

    // You can unbind the VAO afterwards so other VAO calls won't accidentally modify this VAO, but this rarely happens. Modifying other
    // VAOs requires a call to glBindVertexArray anyways so we generally don't unbind VAOs (nor VBOs) when it's not directly necessary.
    glBindVertexArray(0);
}

I don’t know why hard coding it in the ctor works and just passing it in as an argument doesn’t.
Here is what it looks like when I step through it with breakpoints:
Argumented Array
Hard Coded array

I’ve tried changing the way that the arrays are passed in and nothing. I also tried a vector, both passing as argument and hard coding it in, nadda.