A C++ template library for embedded applications
MIT licensed
Designed and
maintained by
John Wellbelove
Support the development
of the ETL

Interfacing with C


Interfacing ETL containers with contiguous storage to a C API is fairly easy to do.
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This is a C function that copies count characters , starting from 'a', to a buffer pointed to by start.

size_t CFunctionCopyToBuffer(char* start, size_t count)
{
  size_t i = 0;
  while (i < count)
  {
    *p++ = 'a' + i;
    ++i;
  }
  return i;
}
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Array

The ETL array is simply a C array wrapped in a C++ interface.

size_t length;
etl::array<char, 26> buffer;
length = CFunctionCopyToBuffer(buffer.data(), buffer.max_size() / 2);

Array contains [ a, b, c, d, e, f, g, h, i, j, k, l, m ]
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Vector

To use a vector with C, you must make room for the expected new data, and then adjust to the new size after copying.

etl::vector<char, 26> buffer;
buffer.uninitialized_resize(buffer.max_size()); // Make the vector as big as it can be.
size_t newLength = CFunctionCopyToBuffer(buffer.data(), buffer.max_size() / 2);
buffer.resize(newLength); // Now adjust it back to the actual new size.

Vector contains [ a, b, c, d, e, f, g, h, i, j, k, l, m ]
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String

Strings are terminated with null characters. We can use this to easily adjust the length after copying.

etl::string<26> buffer;
buffer.initialize_free_space(); // Ensure that the string's free space are terminators characters.
CFunctionCopyToBuffer(buffer.data(), buffer.max_size() / 2);
buffer.trim_to_terminator(); // Trim the string length at the first null terminator.

String contains "abcdefghijklm"

// Lets add some more characters. Start at data_end() so we append to what's already there.
// We'll use the alternative method to adjust the string length.
// We could still use trim_to_terminator() if we wanted to.
size_t oldSize = buffer.size(); // Remember the current size.
buffer.uninitialized_resize(buffer.max_size()); // Make the string as big as it can be.
size_t appendedSize = CFunctionCopyToBuffer(buffer.data_end(), buffer.max_size() / 2);
buffer.uninitialized_resize(oldSize + appendedSize); // Now adjust it back to the actual new size.

String contains "abcdefghijklmabcdefghijklm"