c++ - 模板类实例化中的指针转换无效

Question

我在我的 .h 文件中声明了以下模板类

enum class Hal_uart_id
{
    Uart = 0,
    Usart0,
    Usart1,
    Usart2,
    Usart3,
    UsbUart,
};

template <class T> 
class HalUART
{
    public:
        HalUART(Hal_uart_id uart_id);
    private:
        Hal_uart_id _uart_id;
        T* _p_uart; 
}

在我的 .cpp 文件中，我按如下方式实现类构造函数:

#include "hal_uart_common.h"
#include "Arduino.h"

template <class T> 
HalUART<T>::HalUART(Hal_uart_id id)
{
    _uart_id = id;
    switch (_uart_id)
    {
        case Hal_uart_id::Uart:
            _p_uart = &Serial;
            break;
        case Hal_uart_id::UsbUart:
            _p_uart = &SerialUSB;
            break;
        case Hal_uart_id::Usart0:
            _p_uart = &Serial1;
            break;
        case Hal_uart_id::Usart1:
            _p_uart = &Serial2;
            break;
        case Hal_uart_id::Usart3:
            _p_uart = &Serial3;
            break;
        default:
            break;
    }
}

在我的 .cpp 文件的末尾，我用 USARTClass 类实例化了模板类

template class HalUART<USARTClass>;

我收到以下编译错误，我不明白为什么或如何修复它:

src/hal/uart/hal_uart_sam3x.cpp: In instantiation of 'HalUART<T>::HalUART(Hal_uart_id) [with T = USARTClass]':
src/hal/uart/hal_uart_sam3x.cpp:58:16:   required from here
src/hal/uart/hal_uart_sam3x.cpp:23:21: error: invalid conversion from 'UARTClass*' to 'USARTClass*' [-fpermissive]
             _p_uart = &Serial;
             ~~~~~~~~^~~~~~~~~
src/hal/uart/hal_uart_sam3x.cpp:26:21: error: cannot convert 'Serial_*' to 'USARTClass*' in assignment
             _p_uart = &SerialUSB;
             ~~~~~~~~^~~~~~~~~~~~
*** [.pio/build/due/src/hal/uart/hal_uart_sam3x.cpp.o] Error 1

这些对象在Arduino核心中定义

UARTClass Serial;
USARTClass Serial1;
USARTClass Serial2;
USARTClass Serial3;
Serial_ SerialUSB;

有关 UART/USART 类和对象的定义，请参阅: https://github.com/arduino/ArduinoCore-sam/blob/master/cores/arduino/UARTClass.h https://github.com/arduino/ArduinoCore-sam/blob/master/cores/arduino/UARTClass.cpp

https://github.com/arduino/ArduinoCore-sam/blob/master/cores/arduino/USARTClass.h https://github.com/arduino/ArduinoCore-sam/blob/master/cores/arduino/USARTClass.cpp

https://github.com/arduino/ArduinoCore-sam/blob/master/cores/arduino/USB/USBAPI.h https://github.com/arduino/ArduinoCore-sam/blob/master/cores/arduino/USB/CDC.cpp

Answer 1

模板类型必须在编译时解决。用 template class HalUART<UARTClass>; 实例化你的类而不是 USARTClass (一次 class USARTClass : public UARTClass; )。而且，在 Serial_ 中例如，使用 if constexpr (仅限 C++17)与 std::is_same :

case Hal_uart_id::UsbUart:
    if constexpr (std::is_same_v<T, Serial_>)
        _p_uart = &SerialUSB;
    break;

我推荐使用 if constexpr对于其他情况，一旦你用 Serial_ 实例化了你的类，例如，那么您还可以 template class HalUART<USARTClass>; :

switch (_uart_id)
{
case Hal_uart_id::Uart:
    if constexpr (std::is_same_v<UARTClass, T>)
        _p_uart = &Serial;
    break;
case Hal_uart_id::UsbUart:
    if constexpr (std::is_same_v<T, Serial_>)
        _p_uart = &SerialUSB;
    break;
case Hal_uart_id::Usart0:
    if constexpr (std::is_same_v<USARTClass, T>)
        _p_uart = &Serial1;
    break;
case Hal_uart_id::Usart1:
    if constexpr (std::is_same_v<USARTClass, T>)
        _p_uart = &Serial2;
    break;
case Hal_uart_id::Usart3:
    if constexpr (std::is_same_v<USARTClass, T>)
        _p_uart = &Serial3;
    break;
default:
    break;
}

通过这种类型检查，也许你的 switch可能不再需要。尝试对此进行评估。

考虑使用 std::is_base_of 如有必要。

---

如果 c++17 不可用，请尝试专门化您的函数:

template <class T>
HalUART<T>::HalUART(Hal_uart_id id) {}

template <>
HalUART<UARTClass>::HalUART(Hal_uart_id id) {
    if (id == Hal_uart_id::Uart)
        _p_uart = &Serial;
}
template <>
HalUART<Serial_>::HalUART(Hal_uart_id id) {
    if (id == Hal_uart_id::UsbUart)
        _p_uart = &SerialUSB;
}
template <>
HalUART<USARTClass>::HalUART(Hal_uart_id id) {
    if (id == Hal_uart_id::Usart0)
        _p_uart = &Serial1;
    else if (id == Hal_uart_id::Usart1)
        _p_uart = &Serial2;
    else if (id == Hal_uart_id::Usart3)
        _p_uart = &Serial3;
}

live example