向量Push_Back VS Emplace_Back

Question

我有两种将向量元素添加到向量的方法。

GUI_Vertices.emplace_back();

GUI_Vertices.back().pos.x = ((float)x / 400) - 1.f;
GUI_Vertices.back().pos.y = ((float)y / 300) - 1.f;
GUI_Vertices.back().texCoord.x = u;
GUI_Vertices.back().texCoord.y = v;
GUI_Vertices.back().color.r = m_Color.r / 128;
GUI_Vertices.back().color.g = m_Color.g / 128;
GUI_Vertices.back().color.b = m_Color.b / 128;
GUI_Vertices.back().color.a = m_Color.a / 128;

上面的代码有效，但是我被迫向GUI_Vertices向量添加一个新元素。

Vertex NewVertex;

NewVertex.pos.x = ((float)x / 400) - 1.f;
NewVertex.pos.y = ((float)y / 300) - 1.f;
NewVertex.texCoord.x = u;
NewVertex.texCoord.y = v;
NewVertex.color.r = m_Color.r / 128;
NewVertex.color.g = m_Color.g / 128;
NewVertex.color.b = m_Color.b / 128;
NewVertex.color.a = m_Color.a / 128;

GUI_Vertices.emplace_back(NewVertex);

上面的代码有时可行，如果需要，我可以有条件地将其添加NewVertex到GUI_Vertices向量中。

这是的定义Vertex：

struct Vertex {
    glm::vec3 pos;
    glm::vec4 color;
    glm::vec2 texCoord;

    static VkVertexInputBindingDescription getBindingDescription() {
        VkVertexInputBindingDescription bindingDescription = {};
        bindingDescription.binding = 0;
        bindingDescription.stride = sizeof(Vertex);
        bindingDescription.inputRate = VK_VERTEX_INPUT_RATE_VERTEX;

        return bindingDescription;
    }

    static std::array<VkVertexInputAttributeDescription, 3> getAttributeDescriptions() {
        std::array<VkVertexInputAttributeDescription, 3> attributeDescriptions = {};

        attributeDescriptions[0].binding = 0;
        attributeDescriptions[0].location = 0;
        attributeDescriptions[0].format = VK_FORMAT_R32G32B32_SFLOAT;
        attributeDescriptions[0].offset = offsetof(Vertex, pos);

        attributeDescriptions[1].binding = 0;
        attributeDescriptions[1].location = 1;
        attributeDescriptions[1].format = VK_FORMAT_R32G32B32A32_SFLOAT;
        attributeDescriptions[1].offset = offsetof(Vertex, color);

        attributeDescriptions[2].binding = 0;
        attributeDescriptions[2].location = 2;
        attributeDescriptions[2].format = VK_FORMAT_R32G32_SFLOAT;
        attributeDescriptions[2].offset = offsetof(Vertex, texCoord);

        return attributeDescriptions;
    }

    bool operator==(const Vertex& other) const {
        return pos == other.pos && color == other.color && texCoord == other.texCoord;
    }
};

namespace std {
    template<> struct hash<Vertex> {
        size_t operator()(Vertex const& vertex) const {
            return ((hash<glm::vec3>()(vertex.pos) ^
                (hash<glm::vec4>()(vertex.color) << 1)) >> 1) ^
                (hash<glm::vec2>()(vertex.texCoord) << 1);
        }
    };
}

稍后在程序执行中，将所有Vertex元素添加到GUI_Vertex向量中后，我对进行以下操作GUI_Vertex：

memcpy(GUI_VertexAllocation->GetMappedData(), GUI_Vertices.data(), sizeof(Vertex) * GUI_Vertices.size());

我正在将内存从中复制GUI_Vertices到预分配的缓冲区中，Vulkan将使用该缓冲区来渲染我们的顶点。

现在，我试图弄清楚为什么将Vertex对象添加到第一种方法GUI_Vertices始终有效，而第二种方法有时仅可行。

Here is a link to the entire project https://github.com/kklouzal/WorldEngine/blob/GUI_Indirect_Draw/Vulkan/VulkanGWEN.hpp

After recompiling the project the second method will occasionally work so I'm getting some undefined behavior here. I have checked the validity of GUI_Vertices up until the point where we do our memcpy and the data appears to be valid so I'm not sure whats going on.

I would like to get the second method working so I can conditionally add new vertices into the buffer.

Answer 1

NewVertex.pos.x = ((float)x / 400) - 1.f;
NewVertex.pos.y = ((float)y / 300) - 1.f;
...
glm::vec3 pos;

emplace_back will always perform value initialization on the object it creates, which initializes all of the data members. By contrast, Vertex NewVertex; will default-initialize the object, which leaves its members uninitialized (since the GLM types have trivial default constructors).

So pos.z is uninitialized. And your code doesn't initialize it yourself. So you're sending uninitialized garbage to the GPU.

If you create the object with Vertex NewVertex{};, then it will be value-initialized, just like emplace_back does.