如何在C ++ DLL中使用来自Delphi的Unicode

问题描述

如何在Delphi中使用Unicode PWideChar来在DLL中调用C ++函数?我想将一个字符串从Delphi发送到C ++并进行修改.

How can I use a Unicode PWideChar in Delphi to call a C++ function in a DLL? I want to send a string from Delphi to C++ and modify it.

function Test(a: PWideChar): Integer; cdecl; external 'c:\Win32Project1.dll' name 'Test';

extern "C" __declspec(dllexport) int __cdecl Test(char* a)
{
    a = "汉语";
    return 0;
}

推荐答案

通常，调用方分配一个缓冲区，该缓冲区与缓冲区长度一起传递给被调用方.然后，被调用者填充缓冲区.

Typically the caller allocates a buffer which is passed to the callee, along with the buffer length. The callee then populates the buffer.

size_t Test(wchar_t* buff, const size_t len)
{
    const std::wstring str = ...;
    if (buff != nullptr)
        wcsncpy(buff, str.c_str(), len);
    return str.size()+1; // the length required to copy the string
}

在Delphi方面，您可以这样称呼它:

On the Delphi side you would call it like this:

function Test(buff: PWideChar; len: size_t): size_t; cdecl; external "mydll.dll";
....
var
  buff: array [0..255] of WideChar;
  s: string;
....
Test(buff, Length(buff));
s := buff;

如果您不想分配固定长度的缓冲区，则可以调用该函数以找出需要多大的缓冲区:

If you don't want to allocate a fixed length buffer, then you call the function to find out how large a buffer is needed:

var
  s: string;
  len: size_t;
....
len := Test(nil, 0);
SetLength(s, len-1);
Test(PWideChar(s), len);

如果希望将值传递给函数，建议您通过其他参数来实现.这样可以更方便地调用，并且不会强制您确保输入字符串的缓冲区足够大以容纳输出字符串.也许是这样的:

If you wish to pass a value to the function, I suggest that you do that through a different parameter. That makes it more convenient to call, and does not force you to make sure that the input string has a buffer large enough to admit the output string. Maybe like this:

size_t Test(const wchar_t* input, wchar_t* output, const size_t outlen)
{
    const std::wstring inputStr = input;
    const std::wstring outputStr = foo(inputStr);
    if (buff != nullptr)
        wcsncpy(buff, outputStr.c_str(), len);
    return outputStr.size()+1; // the length required to copy the string
}

另一方面，应该是

function Test(input, output: PWideChar; outlen: size_t): size_t; cdecl;
  external "mydll.dll";

调用代码应该很明显.

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