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Source file src/reflect/makefunc.go

Documentation: reflect

     1  // Copyright 2012 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // MakeFunc implementation.
     6  
     7  package reflect
     8  
     9  import (
    10  	"unsafe"
    11  )
    12  
    13  // makeFuncImpl is the closure value implementing the function
    14  // returned by MakeFunc.
    15  // The first two words of this type must be kept in sync with
    16  // methodValue and runtime.reflectMethodValue.
    17  // Any changes should be reflected in all three.
    18  type makeFuncImpl struct {
    19  	code  uintptr
    20  	stack *bitVector
    21  	typ   *funcType
    22  	fn    func([]Value) []Value
    23  }
    24  
    25  // MakeFunc returns a new function of the given Type
    26  // that wraps the function fn. When called, that new function
    27  // does the following:
    28  //
    29  //	- converts its arguments to a slice of Values.
    30  //	- runs results := fn(args).
    31  //	- returns the results as a slice of Values, one per formal result.
    32  //
    33  // The implementation fn can assume that the argument Value slice
    34  // has the number and type of arguments given by typ.
    35  // If typ describes a variadic function, the final Value is itself
    36  // a slice representing the variadic arguments, as in the
    37  // body of a variadic function. The result Value slice returned by fn
    38  // must have the number and type of results given by typ.
    39  //
    40  // The Value.Call method allows the caller to invoke a typed function
    41  // in terms of Values; in contrast, MakeFunc allows the caller to implement
    42  // a typed function in terms of Values.
    43  //
    44  // The Examples section of the documentation includes an illustration
    45  // of how to use MakeFunc to build a swap function for different types.
    46  //
    47  func MakeFunc(typ Type, fn func(args []Value) (results []Value)) Value {
    48  	if typ.Kind() != Func {
    49  		panic("reflect: call of MakeFunc with non-Func type")
    50  	}
    51  
    52  	t := typ.common()
    53  	ftyp := (*funcType)(unsafe.Pointer(t))
    54  
    55  	// Indirect Go func value (dummy) to obtain
    56  	// actual code address. (A Go func value is a pointer
    57  	// to a C function pointer. https://golang.org/s/go11func.)
    58  	dummy := makeFuncStub
    59  	code := **(**uintptr)(unsafe.Pointer(&dummy))
    60  
    61  	// makeFuncImpl contains a stack map for use by the runtime
    62  	_, _, _, stack, _ := funcLayout(t, nil)
    63  
    64  	impl := &makeFuncImpl{code: code, stack: stack, typ: ftyp, fn: fn}
    65  
    66  	return Value{t, unsafe.Pointer(impl), flag(Func)}
    67  }
    68  
    69  // makeFuncStub is an assembly function that is the code half of
    70  // the function returned from MakeFunc. It expects a *callReflectFunc
    71  // as its context register, and its job is to invoke callReflect(ctxt, frame)
    72  // where ctxt is the context register and frame is a pointer to the first
    73  // word in the passed-in argument frame.
    74  func makeFuncStub()
    75  
    76  // The first two words of this type must be kept in sync with
    77  // makeFuncImpl and runtime.reflectMethodValue.
    78  // Any changes should be reflected in all three.
    79  type methodValue struct {
    80  	fn     uintptr
    81  	stack  *bitVector
    82  	method int
    83  	rcvr   Value
    84  }
    85  
    86  // makeMethodValue converts v from the rcvr+method index representation
    87  // of a method value to an actual method func value, which is
    88  // basically the receiver value with a special bit set, into a true
    89  // func value - a value holding an actual func. The output is
    90  // semantically equivalent to the input as far as the user of package
    91  // reflect can tell, but the true func representation can be handled
    92  // by code like Convert and Interface and Assign.
    93  func makeMethodValue(op string, v Value) Value {
    94  	if v.flag&flagMethod == 0 {
    95  		panic("reflect: internal error: invalid use of makeMethodValue")
    96  	}
    97  
    98  	// Ignoring the flagMethod bit, v describes the receiver, not the method type.
    99  	fl := v.flag & (flagRO | flagAddr | flagIndir)
   100  	fl |= flag(v.typ.Kind())
   101  	rcvr := Value{v.typ, v.ptr, fl}
   102  
   103  	// v.Type returns the actual type of the method value.
   104  	funcType := v.Type().(*rtype)
   105  
   106  	// Indirect Go func value (dummy) to obtain
   107  	// actual code address. (A Go func value is a pointer
   108  	// to a C function pointer. https://golang.org/s/go11func.)
   109  	dummy := methodValueCall
   110  	code := **(**uintptr)(unsafe.Pointer(&dummy))
   111  
   112  	// methodValue contains a stack map for use by the runtime
   113  	_, _, _, stack, _ := funcLayout(funcType, nil)
   114  
   115  	fv := &methodValue{
   116  		fn:     code,
   117  		stack:  stack,
   118  		method: int(v.flag) >> flagMethodShift,
   119  		rcvr:   rcvr,
   120  	}
   121  
   122  	// Cause panic if method is not appropriate.
   123  	// The panic would still happen during the call if we omit this,
   124  	// but we want Interface() and other operations to fail early.
   125  	methodReceiver(op, fv.rcvr, fv.method)
   126  
   127  	return Value{funcType, unsafe.Pointer(fv), v.flag&flagRO | flag(Func)}
   128  }
   129  
   130  // methodValueCall is an assembly function that is the code half of
   131  // the function returned from makeMethodValue. It expects a *methodValue
   132  // as its context register, and its job is to invoke callMethod(ctxt, frame)
   133  // where ctxt is the context register and frame is a pointer to the first
   134  // word in the passed-in argument frame.
   135  func methodValueCall()
   136  

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