为什么我不能像这样在 QMap 上调用 insert ？

发布于 2025-01-03 23:37:42 字数 1366 浏览 4 评论 0 原文

尝试在 Qt SDK 4.7.4 for Desktop - MinGW 4.4 下编译以下代码会导致以下编译器错误：

#include <QtCore/QCoreApplication>
#include <QMap>
struct Buffer
{
   char data[4];
};
// A Bucket needs to reserve 16 chars worth of Buffers
typedef Buffer Bucket[(16 * (sizeof (char))) / (sizeof (Buffer))];
typedef QMap<QString, Bucket *> BucketMap;
int main(int argc, char *argv[])
{
   BucketMap bucket;
   bucket.insert(QString("foo"), new Bucket()); //compile error
   return 0;
}

../test/main.cpp: In function 'int main(int, char**)':
../test/main.cpp:13: error: no matching function for call to 'QMap<QString, Buffer (*)[4]>::insert(QString, Buffer*)'
../../../QtSDK/Desktop/Qt/4.7.4/mingw/include/QtCore/qmap.h:556: note: candidates are: QMap<Key, T>::iterator QMap<Key, T>::insert(const Key&, const T&) [with Key = QString, T = Buffer (*)[4]]
mingw32-make.exe[1]: *** [debug/main.o] Error 1
mingw32-make.exe: *** [debug] Error 2

我尝试使用 std::string 和 std 将其转换为等效示例::map 达到相同的效果。我提供了 Qt 版本，因为它更紧凑，并且最终是我的项目所需的形式。
我猜我只是错过了有关 typedef 最终如何解释的一些内容。为什么 insert 显然是一个 Buffer * （不是 Buffer(*)[4]），我该如何修复它？

原文

Attempting to compile the following code under Qt SDK 4.7.4 for Desktop - MinGW 4.4 results in the compiler error below:

#include <QtCore/QCoreApplication>
#include <QMap>
struct Buffer
{
   char data[4];
};
// A Bucket needs to reserve 16 chars worth of Buffers
typedef Buffer Bucket[(16 * (sizeof (char))) / (sizeof (Buffer))];
typedef QMap<QString, Bucket *> BucketMap;
int main(int argc, char *argv[])
{
   BucketMap bucket;
   bucket.insert(QString("foo"), new Bucket()); //compile error
   return 0;
}

../test/main.cpp: In function 'int main(int, char**)':
../test/main.cpp:13: error: no matching function for call to 'QMap<QString, Buffer (*)[4]>::insert(QString, Buffer*)'
../../../QtSDK/Desktop/Qt/4.7.4/mingw/include/QtCore/qmap.h:556: note: candidates are: QMap<Key, T>::iterator QMap<Key, T>::insert(const Key&, const T&) [with Key = QString, T = Buffer (*)[4]]
mingw32-make.exe[1]: *** [debug/main.o] Error 1
mingw32-make.exe: *** [debug] Error 2

I have tried converting this to an quivalent example using std::string and std::map to the same effect. I have presented the Qt version because it is more compact and ultimately the form that my project requires.
I'm guessing that I am simply missing something about how the typedef is ultimately interpreted. Why is the second argument to insert apparently a Buffer * (not Buffer(*)[4]), and how can I fix it?

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对你再特殊 2025-01-10 23:37:42

简单回答：类型不匹配。

您实际上需要了解的是：您不能为数组类型调用 new 表达式。因此，以下两个是不等价的（并且第一个是不合法的）：

typedef T TArr[4]; TArr * p = new TArr;  // #1

T * q = new T[4];                        // #2

该语言不能那样工作。第二个版本创建一个动态数组，而第一个版本想要创建“4 T 数组”类型的单个动态对象，这是不可能的。相反，new TArr 实际上与 new T[4] 相同，因此结果是一个由四个 T 组成的动态数组 s.*

您基本上应该将地图的值类型更改为 Buffer * 或 std::array; * 或 std::unique_ptr 或 std::unique_ptr>，无论您选择哪个更喜欢。

_{*) 这正是以下代码非常有问题的原因： template ; void foo() { T * p = 新 T;删除p； } 想象你说 foo();...}

Simple answer: The types don't match.

What you actually need to know: You cannot invoke a new expression for an array type. Therefore, the following two are not equivalent (and the first isn't legal):

typedef T TArr[4]; TArr * p = new TArr;  // #1

T * q = new T[4];                        // #2

The language just doesn't work that way. The second version creates a dynamic array, while the first version would like to create a single dynamic object of type "array of 4 T", which is not possible. Instead, new TArr is actually the same as new T[4], and so the result is a dynamic array of four Ts.*

You should basically just change your map's value type to either Buffer * or std::array<Buffer, 4> * or std::unique_ptr<Buffer[]> or std::unique_ptr<std::array<Buffer, 4>>, whichever you prefer.

_{*) This is exactly the reason why the following code is very problematic: template <T> void foo() { T * p = new T; delete p; } Imagine you say foo<int[4]>();...}