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I'm writing a wrapper around a C mathematical library. Every function takes one or two functions as arguments. However, the arguments for those child functions (as well as the parent functions) are not Swifty -hence the wrapper.

I've cleaned up the example code to just show the three main pieces: the c-library function, the desired Swift function that would be passed to the wrapper (body not shown, but wrapping around the c-library function), and the required C function form.

//C library function, that calls the passed function dozens, hundreds or thousands of times, each time it changes the data provided in p, and uses the output from x
//The Swift arrays are passed as pointers, and the length of the and x array are m and n respectively
returnValue = cLibraryFunc(passedFunc, &p, &x, Int32(m), Int32(n), Int32(itmax), &opts, &info, &work, &covar, &adata)


//I would like to create a Swift function that would look like this (internals could be any myriad of things that takes inputs p and adata and returns data in x:
func desiredSwifty(p: inout [Double], x: inout [Double], m: Int, n: Int, adata: inout [Double]) {
    //very simple example
    //this example knows the length of p (so m as well)
    //and assumes that adata length is the same as the x length (n)
    //obviously, it could ifer m and n from p.count and x.count

    for i in 0..<n {
        x[i] = p[0] + p[1]*adata[i]  + p[2]*pow(adata[i], 2)
    }
}


//And the wrapper would "convert" it -internally- into the form that the C library function requires:
func requiredC(p: UnsafeMutablePointer<Double>?, x: UnsafeMutablePointer<Double>?, m: Int32, n: Int32, adata: UnsafeMutablePointer<Void>?) {
    //same thing, but using pointers, and uglier

    //first, have to bitcast the void back to a double
    let adataDouble : UnsafeMutablePointer<Double> = unsafeBitCast(adata, to: UnsafeMutablePointer<Double>.self)

    for i in 0..<Int(n) {
        x![i] = p![0] + p![1]*adataDouble[i]  + p![2]*pow(adataDouble[i], 2)
    }
}

addition

I should add that I have access to the c source code, so I could possibly add some dummy parameters (possibly to find a way to pass context in). But given that the docs seem to indicate that one can't grab context with a c function pointer, this may be of no use.

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2 回答 2

5

（注意：以下示例在 Xcode 8 beta 2 上使用 Swift 3。）

您的问题是关于将另一个 C 函数作为参数的 C 函数，所以让我们将问题简化为该问题。这是一个简单的 C 函数，它接受一个参数，它又是一个 C 函数，它接受一个指向双精度数组和整数计数的指针：

// cfunction.h:
void cFunc(void (*func)(double *values, int count));

// cfunction.c:
void cFunc(void (*func)(double *values, int count)) {
    double x[] = { 1.2, 3.4, 5,6 };
    func(x, 3);
}

这个函数被导入到 Swift 作为

func cFunc(_ func: (@convention(c) (UnsafeMutablePointer<Double>?, Int32) -> Swift.Void)!)

这里@convention(c)声明块具有 C 风格的调用约定。特别是，在 Swift 中，您只能传递一个全局函数或一个不捕获任何上下文的闭包。

Swift 包装器的一个简单示例是

func swiftyFunc(passedFunc: (@convention(c) (UnsafeMutablePointer<Double>?, Int32) -> Void)) {
    cFunc(passedFunc) 
}

您可以像这样使用它：

func functionToPass(values: UnsafeMutablePointer<Double>?, count: Int32) {
    let bufPtr = UnsafeBufferPointer(start: values, count: Int(count))
    for elem in bufPtr { print(elem) }
}

swiftyFunc(passedFunc: functionToPass)

或带有闭包参数：

swiftyFunc { (values, count) in
    let bufPtr = UnsafeBufferPointer(start: values, count: Int(count))
    for elem in bufPtr { print(elem) }
}
于 2016-07-12T16:33:55.020 回答
0

你知道你可以var通过使用&操作符来获得一个指向 a 的可变指针吗？它也在数组上做“正确的事情”。

func foo(_ x: UnsafeMutablePointer<Int>) {
    print(x)
}

func bar(_ x: UnsafeMutablePointer<Int>) {
    print(x)
}

var array = [0]
foo(&array)

var int = 0
bar(&int)

（在 Swift 2 上测试，但很可能在 Swift 3 上仍然有效。）

我怀疑这可能会大大减少您对包装器的需求。

于 2016-07-12T20:48:35.913 回答