A typemap is created for each type to describe to Shroud how it should convert a type between languages for each wrapper. Native types are predefined and a Shroud typemap is created for each struct and class declaration.

The general form is:

- type: type-name

type-name is the name used by C++. There are some fields which are used by all wrappers and other fields which are used by language specific wrappers.

type fields

These fields are common to all wrapper languages.


The base type of type-name. This is used to generalize operations for several types. The base types that Shroud uses are string, vector, or shadow.


A c preprocessor test which is used to conditionally use other fields of the type such as c_header and cxx_header:

- type: MPI_Comm
    cpp_if: ifdef USE_MPI


A flattened version of cxx_type which allows the name to be used as a legal identifier in C, Fortran and Python. By default any scope separators are converted to underscores i.e. internal::Worker becomes internal_Worker. Imbedded blanks are converted to underscores i.e. unsigned int becomes unsigned_int. And template arguments are converted to underscores with the trailing > being replaced i.e. std::vector<int> becomes std_vector_int.

One use of this name is as the function_suffix for templated functions.


Index of capsule_data destructor in the function C_memory_dtor_function. This value is computed by Shroud and should not be set. It can be used when formatting statements as {idtor}. Defaults to 0 indicating no destructor.


Override fields when result should be treated as an argument. Defaults to None.


Each language also provides a section that is used to insert language specific statements into the wrapper. These are named c_statements, f_statements, and py_statements.

The are broken down into several resolutions. The first is the intent of the argument. result is used as the intent for function results.

Code to add for argument with intent(IN). Can be used to convert types or copy-in semantics. For example, char * to std::string.
Code to add after call when intent(OUT). Used to implement copy-out semantics.
Code to add after call when intent(INOUT). Used to implement copy-out semantics.
Result of function. Including when it is passed as an argument, F_string_result_as_arg.

Each intent is then broken down into code to be added into specific sections of the wrapper. For example, declaration, pre_call and post_call.

Each statement is formatted using the format dictionary for the argument. This will define several variables.

The C name of the argument.
Name of the C++ variable.
Fortran variable name for argument.

For example:

  - '{c_var} = {f_var}  ! coerce to C_BOOL'
  - '{f_var} = {c_var}  ! coerce to logical'

Note that the code lines are quoted since they begin with a curly brace. Otherwise YAML would interpret them as a dictionary.

See the language specific sections for details.

Numeric Types

The numeric types usually require no conversion. In this case the type map is mainly used to generate declaration code for wrappers:

type: int
    c_type: int
    cxx_type: int
    f_type: integer(C_INT)
    f_kind: C_INT
        - C_INT
    f_cast: int({f_var}, C_INT)

One case where a conversion is required is when the Fortran argument is one type and the C++ argument is another. This may happen when an overloaded function is generated so that a C_INT or C_LONG argument may be passed to a C++ function function expecting a long. The f_cast field is used to convert the argument to the type expected by the C++ function.


The first thing to notice is that f_c_type is defined. This is the type used in the Fortran interface for the C wrapper. The type is logical(C_BOOL) while f_type, the type of the Fortran wrapper argument, is logical.

The f_statements section describes code to add into the Fortran wrapper to perform the conversion. c_var and f_var default to the same value as the argument name. By setting c_local_var, a local variable is generated for the call to the C wrapper. It will be named SH_{f_var}.

There is no Fortran intrinsic function to convert between default logical and logical(C_BOOL). The pre_call and post_call sections will insert an assignment statement to allow the compiler to do the conversion.

If a function returns a bool result then a wrapper is always needed to convert the result. The result section sets need_wrapper to force the wrapper to be created. By default a function with no argument would not need a wrapper since there will be no pre_call or post_call code blocks. Only the C interface would be required since Fortran could call the C function directly.

See example checkBool.


Any C++ function which has char or std::string arguments or result will create an additional C function which include additional arguments for the length of the strings. Most Fortran compiler use this convention when passing CHARACTER arguments. Shroud makes this convention explicit for three reasons:

  • It allows an interface to be used. Functions with an interface will not pass the hidden, non-standard length argument, depending on compiler.
  • It may pass the result of len and/or len_trim. The convention just passes the length.
  • Returning character argument from C to Fortran is non-portable.

Arguments with the intent(in) annotation are given the len_trim annotation. The assumption is that the trailing blanks are not part of the data but only padding. Return values and intent(out) arguments add a len annotation with the assumption that the wrapper will copy the result and blank fill the argument so it need to know the declared length.

The additional function will be named the same as the original function with the option C_bufferify_suffix appended to the end. The Fortran wrapper will use the original function name, but call the C function which accepts the length arguments.

The character type maps use the c_statements section to define code which will be inserted into the C wrapper. intent_in, intent_out, and result subsections add actions for the C wrapper. intent_in_buf, intent_out_buf, and result_buf are used for arguments with the len and len_trim annotations in the additional C wrapper.

There are occasions when the bufferify wrapper is not needed. For example, when using char * to pass a large buffer. It is better to just pass the address of the argument instead of creating a copy and appending a NULL. The F_create_bufferify_function options can set to false to turn off this feature.


Ndest is the declared length of argument dest and Lsrc is the trimmed length of argument src. These generated names must not conflict with any other arguments. There are two ways to set the names. First by using the options C_var_len_template and C_var_trim_template. This can be used to control how the names are generated for all functions if set globally or just a single function if set in the function’s options. The other is by explicitly setting the len and len_trim annotations which only effect a single declaration.

The pre_call code creates space for the C strings by allocating buffers with space for an additional character (the NULL). The intent(in) string copies the data and adds an explicit terminating NULL. The function is called then the post_call section copies the result back into the dest argument and deletes the scratch space. ShroudStrCopy is a function provided by Shroud which copies character into the destination up to Ndest characters, then blank fills any remaining space.


MPI_Comm is provided by Shroud and serves as an example of how to wrap a non-native type. MPI provides a Fortran interface and the ability to convert MPI_comm between Fortran and C. The type map tells Shroud how to use these routines:

type: MPI_Comm
    cxx_type: MPI_Comm
    c_header: mpi.h
    c_type: MPI_Fint
    f_type: integer
    f_kind: C_INT
    f_c_type: integer(C_INT)
          - C_INT
    cxx_to_c: MPI_Comm_c2f({cxx_var})
    c_to_cxx: MPI_Comm_f2c({c_var})

This mapping makes the assumption that integer and integer(C_INT) are the same type.


Shroud will wrap templated classes and functions for explicit instantiations. The template is given as part of the decl and the instantations are listed in the cxx_template section:

- decl: |
      template<typename ArgType>
      void Function7(ArgType arg)
  - instantiation: <int>
  - instantiation: <double>

options and format may be provide to control the generated code:

- decl: template<typename T> class vector
  cxx_header: <vector>
  - instantiation: <int>
      C_impl_filename: wrapvectorforint.cpp
      optblah: two
  - instantiation: <double>

For a class template, the class_name is modified to included the instantion type. If only a single template parameter is provided, then the template argument is used. For the above example, C_impl_filename will default to wrapvector_int.cpp but has been explicitly changed to wrapvectorforint.cpp.

Memory Management

Shroud will maintain ownership of memory via the owner attribute. It uses the value of the attribute to decided when to release memory.

Use owner(library) when the library owns the memory and the user should not release it. For example, this is used when a function returns const std::string & for a reference to a string which is maintained by the library. Fortran and Python will both get the reference, copy the contents into their own variable (Fortran CHARACTER or Python str), then return without releasing any memory. This is the default behavior.

Use owner(caller) when the library allocates new memory which is returned to the caller. The caller is then responsible to release the memory. Fortran and Python can both hold on to the memory and then provide ways to release it using a C++ callback when it is no longer needed.

For shadow classes with a destructor defined, the destructor will be used to release the memory.

The c_statements may also define a way to destroy memory. For example, std::vector provides the lines:

destructor_name: std_vector_{cxx_T}
-  std::vector<{cxx_T}> *cxx_ptr = reinterpret_cast<std::vector<{cxx_T}> *>(ptr);
-  delete cxx_ptr;

Patterns can be used to provide code to free memory for a wrapped function. The address of the memory to free will be in the variable void *ptr, which should be referenced in the pattern:

- decl: char * getName() +free_pattern(free_getName)

   free_getName: |

Without any explicit destructor_name or pattern, free will be used to release POD pointers; otherwise, delete will be used.

C and Fortran

Fortran keeps track of C++ objects with the struct C_capsule_data_type and the bind(C) equivalent F_capsule_data_type. Their names default to {C_prefix}SHROUD_capsule_data and SHROUD_{F_name_scope}capsule. In the Tutorial these types are defined in typesTutorial.h as:

And wrapftutorial.f:

addr is the address of the C or C++ variable, such as a char * or std::string *. idtor is a Shroud generated index of the destructor code defined by destructor_name or the free_pattern attribute. These code segments are collected and written to function C_memory_dtor_function. A value of 0 indicated the memory will not be released and is used with the owner(library) attribute. A typical function would look like:

// Release library allocated memory.
void TUT_SHROUD_memory_destructor(TUT_SHROUD_capsule_data *cap)
    void *ptr = cap->addr;
    switch (cap->idtor) {
    case 0:   // --none--
        // Nothing to delete
    case 1:   // new_string
        std::string *cxx_ptr = reinterpret_cast<std::string *>(ptr);
        delete cxx_ptr;
        // Unexpected case in destructor
    cap->addr = NULL;
    cap->idtor = 0;  // avoid deleting again

Character and Arrays

In order to create an allocatable copy of a C++ pointer, an additional structure is involved. For example, getConstStringPtrAlloc returns a pointer to a new string. From strings.yaml:

- decl: const std::string * getConstStringPtrAlloc() +owner(library)

The C wrapper calls the function and saves the result along with metadata consisting of the address of the data within the std::string and its length. The Fortran wrappers allocates its return value to the proper length, then copies the data from the C++ variable and deletes it.

The metadata for variables are saved in the C struct C_array_type and the bind(C) equivalent F_array_type.:

struct s_STR_SHROUD_array {
    STR_SHROUD_capsule_data cxx;      /* address of C++ memory */
    union {
        const void * base;
        const char * ccharp;
    } addr;
    int type;        /* type of element */
    size_t elem_len; /* bytes-per-item or character len in c++ */
    size_t size;     /* size of data in c++ */
typedef struct s_STR_SHROUD_array STR_SHROUD_array;

The union for addr makes some assignments easier and also aids debugging. The union is replaced with a single type(C_PTR) for Fortran:

type, bind(C) :: SHROUD_array
    ! address of C++ memory
    type(SHROUD_capsule_data) :: cxx
    ! address of data in cxx
    type(C_PTR) :: base_addr = C_NULL_PTR
    ! type of element
    integer(C_INT) :: type
    ! bytes-per-item or character len of data in cxx
    integer(C_SIZE_T) :: elem_len = 0_C_SIZE_T
    ! size of data in cxx
    integer(C_SIZE_T) :: size = 0_C_SIZE_T
end type SHROUD_array

The C wrapper does not return a std::string pointer. Instead it passes in a C_array_type pointer as an argument. It calls getConstStringPtrAlloc, saves the results and metadata into the argument. This allows it to be easily accessed from Fortran. Since the attribute is owner(library), cxx.idtor is set to 0 to avoid deallocating the memory.

void STR_get_const_string_ptr_alloc_bufferify(STR_SHROUD_array *DSHF_rv)
    // splicer begin function.get_const_string_ptr_alloc_bufferify
    const std::string * SHCXX_rv = getConstStringPtrAlloc();
    ShroudStrToArray(DSHF_rv, SHCXX_rv, 0);
    // splicer end function.get_const_string_ptr_alloc_bufferify

The Fortran wrapper uses the metadata to allocate the return argument to the correct length:

function get_const_string_ptr_alloc() &
    type(SHROUD_array) :: DSHF_rv
    character(len=:), allocatable :: SHT_rv
    ! splicer begin function.get_const_string_ptr_alloc
    call c_get_const_string_ptr_alloc_bufferify(DSHF_rv)
    allocate(character(len=DSHF_rv%elem_len):: SHT_rv)
    call SHROUD_copy_string_and_free(DSHF_rv, SHT_rv, DSHF_rv%elem_len)
    ! splicer end function.get_const_string_ptr_alloc
end function get_const_string_ptr_alloc

Finally, the helper function SHROUD_copy_string_and_free is called to set the value of the result and possible free memory for owner(caller) or intermediate values:

// Copy the char* or std::string in context into c_var.
// Called by Fortran to deal with allocatable character.
void STR_ShroudCopyStringAndFree(STR_SHROUD_array *data, char *c_var, size_t c_var_len) {
    const char *cxx_var = data->addr.ccharp;
    size_t n = c_var_len;
    if (data->elem_len < n) n = data->elem_len;
    std::strncpy(c_var, cxx_var, n);
    STR_SHROUD_memory_destructor(&data->cxx); // delete data->cxx.addr


The three steps of call, allocate, copy could be replaced with a single call by using the futher interoperability with C features of Fortran 2018 (a.k.a TS 29113). This feature allows Fortran ALLOCATABLE variables to be allocated by C. However, not all compilers currently support that feature. The current Shroud implementation works with Fortran 2003.


NumPy arrays control garbage collection of C++ memory by creating a PyCapsule as the base object of NumPy objects. Once the final reference to the NumPy array is removed, the reference count on the PyCapsule is decremented. When 0, the destructor for the capsule is called and releases the C++ memory. This technique is discussed at [blog1] and [blog2]



C_finalize is replaced by statement.final

Shroud generated C wrappers do not explicitly delete any memory. However a destructor may be automatically called for some C++ stl classes. For example, a function which returns a std::string will have its value copied into Fortran memory since the function’s returned object will be destroyed when the C++ wrapper returns. If a function returns a char * value, it will also be copied into Fortran memory. But if the caller of the C++ function wants to transfer ownership of the pointer to its caller, the C++ wrapper will leak the memory.

The C_finalize variable may be used to insert code before returning from the wrapper. Use C_finalize_buf for the buffer version of wrapped functions.

For example, a function which returns a new string will have to delete it before the C wrapper returns:

std::string * getConstStringPtrLen()
    std::string * rv = new std::string("getConstStringPtrLen");
    return rv;

Wrapped as:

- decl: const string * getConstStringPtrLen+len=30()
    C_finalize_buf: delete {cxx_var};

The C buffer version of the wrapper is:

void STR_get_const_string_ptr_len_bufferify(char * SHF_rv, int NSHF_rv)
    const std::string * SHCXX_rv = getConstStringPtrLen();
    if (SHCXX_rv->empty()) {
        std::memset(SHF_rv, ' ', NSHF_rv);
    } else {
        ShroudStrCopy(SHF_rv, NSHF_rv, SHCXX_rv->c_str());
        // C_finalize
        delete SHCXX_rv;

The unbuffer version of the function cannot destroy the string since only a pointer to the contents of the string is returned. It would leak memory when called:

const char * STR_get_const_string_ptr_len()
    const std::string * SHCXX_rv = getConstStringPtrLen();
    const char * SHC_rv = SHCXX_rv->c_str();
    return SHC_rv;


Reference counting and garbage collection are still a work in progress