`std::string` allocations are my current bottleneck - how can I optimize with a custom allocator?

Question

I\'m writing a C++14 JSON library as an exercise and to use it in my personal projects.

By using callgrind I\'ve discovered that the current bottleneck

Answer 1

The best way to avoid a memory allocation is don't do it!
BUT if I remember JSON correctly all the readStr values either gets used as keys or as identifiers so you will have to allocate them eventually, std::strings move semantics should insure that the allocated array are not copied around but reused until its final use. The default NRVO/RVO/Move should reduce any copying of the data if not of the string header itself.

Method 1:
Pass result as a ref from the caller which has reserved SomeResonableLargeValue chars, then clear it at the start of readStr. This is only usable if the caller actually can reuse the string.

Method 2:
Use the stack.

// Reserve memory for the string (BOTTLENECK)
if (end - idx < SomeReasonableValue) { // 32?
  char result[SomeReasonableValue] = {0};  // feel free to use std::array if you want bounds checking, but the preceding "if" should insure its not a problem.
  int ridx = 0;

  for(; idx < end; ++idx) {
    // Not an escape sequence
    if(!isC('\\')) { result[ridx++] = getC(); continue; }
    // Escape sequence: skip '\'
    ++idx;
    // Convert escape sequence
    result[ridx++] = getEscapeSequence(getC());
  }

  // Skip closing '"'
  ++idx;
  result[ridx] = 0; // 0-terminated.
  // optional assert here to insure nothing went wrong.
  return result; // the bottleneck might now move here as the data is copied to the receiving string.
}
// fallback code only if the string is long.
// Your original code here

Method 3:
If your string by default can allocate some size to fill its 32/64 byte boundary, you might want to try to use that, construct result like this instead in case the constructor can optimize it.

Str result(end - idx, 0);

Method 4:
Most systems already has some optimized allocator that like specific block sizes, 16,32,64 etc.

siz = ((end - idx)&~0xf)+16; // if the allocator has chunks of 16 bytes already.
Str result(siz);

Method 5:
Use either the allocator made by google or facebooks as global new/delete replacement.