Question

我想更新一个struct的实例，我将其存储在循环中的map中，但实例变量的更改不会在循环的迭代中生效（在一次迭代，新变量得到适当设置，在下一个操作中，它们被重置为初始值。）

以下是我正在做的简化版本：

map<int, RegionOverFrames>* foundRegions = new map<int, RegionOverFrames>;

for (int i = 0; i < frames.size(); i++) {

    // find all regions in current frame
    map<int, RegionOverFrames> regionsInCurrentFrame;
    for (Region region: currentFrame.regions) {
        if (foundRegions->count(region.regionId) == 0) {
            RegionOverFrames foundRegion;
            foundRegion.regionId = region.regionId;
            regionsInCurrentFrame[region.regionId] = foundRegion;
            (*foundRegions)[region.regionId] = foundRegion;
        }
        else if (foundRegions->count(region.regionId) > 0) {
            RegionOverFrames foundRegion = (*foundRegions)[region.regionId];
            regionsInCurrentFrame[region.regionId] = foundRegion;
        }
    }

    // update found regions (either by adding weight or setting the end index)
    for (auto it = foundRegions->begin(); it != foundRegions->end(); it++) {
        RegionOverFrames foundRegion = it->second;
        // the region that was found before is also present in this frame
        if (regionsInCurrentFrame.count(foundRegion.regionId) > 0) {
            float weight = currentFrame.getRegion(foundRegion.regionId).getWeight();
            foundRegion.accumulatedWeight += weight; // this update of accumulatedWeight is not present in the next loop, the accumulatedWeight only gets assigned the value of weight here, but in the next iteration it's reset to 0
        }
    }
}

这是否与我使用迭代器it访问map<int, RegionOverFrames>* foundRegions中的对象或使用指针声明foundRegions的事实有关存储在堆上？

注意RegionOverFrames是一个简单的struct，如下所示：

struct RegionOverFrames {
   int regionId;
   double accumulatedWeight;
}

Answer 1

您的问题是您正在创建找到的区域的副本，而不是更新地图中找到的对象。

RegionOverFrames foundRegion = it->second;
//               ^ copy created

您应该使用引用：

RegionOverFrames &foundRegion = it->second;
//               ^ use reference

为什么我的变量的更改在下一次迭代之前不会存在？

1 个答案: