Chapter 9. Aggregates
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9.1 Strings
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9.1.1 Literals and Simple Operations
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Another simple operation is concatenation, often represented by the overloaded operator + . (SNOBOL represents concatenation by an empty space operator, which is quite confusing, particularly since the same invisible operator also represents pattern matching!) In addition, a few languages, such as ABC, provide operators for string repetition ("ho" * 3 is "hohoho"), string length, and arcane operations such as finding the minimum character in a string.
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9.1.6 SNOBOL
SNOBOL was developed by Ralph E. Griswold and others at Bell Telephone Laboratories around 1965 [Griswold 71]. It has a strange syntax, partially because it was developed before Algol-like syntax became popular. Spaces act as both the concatenation and the match operators.

String Concatenation
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The D Way
D overloads the operators ~ and ~= for char and wchar arrays to mean concatenate and append, respectively:

char[] s1;
char[] s2;
char[] s;

s = s1 ~ s2;
s ~= "hello";

Array Concatenation

The binary operator ~ is the cat operator. It is used to concatenate arrays:

int[] a;
int[] b;
int[] c;

a = b ~ c; // Create an array from the concatenation of the
           // b and c arrays

Many languages overload the + operator to mean concatenation. This confusingly leads to, does:

"10" + 3

produce the number 13 or the string "103" as the result? It isn't obvious, and the language designers wind up carefully writing rules to disambiguate it — rules that get incorrectly implemented, overlooked, forgotten, and ignored. It's much better to have + mean addition, and a separate operator to be array concatenation.

Similarly, the ~= operator means append, as in:

a ~= b; // a becomes the concatenation of a and b

Concatenation always creates a copy of its operands, even if one of the operands is a 0 length array, so:

a = b;           // a refers to b
a = b ~ c[0..0]; // a refers to a copy of b

Concatenation Operator

C++ Strings are stuck with overloading existing operators. The obvious choice for concatenation is += and +. But someone just looking at the code will see + and think "addition". He'll have to look up the types (and types are frequently buried behind multiple typedef's) to see that it's a string type, and it's not adding strings but concatenating them.

Additionally, if one has an array of floats, is '+' overloaded to be the same as a vector addition, or an array concatenation?

In D, these problems are avoided by introducing a new binary operator ~ as the concatenation operator. It works with arrays (of which strings are a subset). ~= is the corresponding append operator. ~ on arrays of floats would concatenate them, + would imply a vector add. Adding a new operator makes it possible for orthogonality and consistency in the treatment of arrays. (In D, strings are simply arrays of characters, not a special type.)

ПС>"10" + 3
ПС>

#include <iostream>

int main()
{
  std::cout << "10" + 3;
  return 0;
}

std::string operator*(const std::string& src, int i)
{
  std::string ret;
  ret.reserve( i*src.length() );
  while (i--) ret.append( src );
  return ret;
}

template <class Container>
Container operator*(const Container& src, int i)
{
  Container ret;
  while (i--)
    ret.insert( ret.end(), src.begin(), src.end() );
  return ret;
}

Глава 3. Лексический анализ
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3.3. Определение токенов
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Строки и языки
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Пустая строка, обозначаемая как ε, представляет собой специальную строку нулевой длины.
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Если x и y — строки, то конкатенация строк x и y, которая записывается как xy, является строкой, сформированной путём дописывания y к x. Например, если x = dog, а y = house, то xy = doghouse. Пустая строка представляет собой единичный элемент по отношению к операции конкатенации, то есть для любой строки s справедливо соотношение sε = εs = s.

Если рассматривать конкатенацию как операцию «умножения», то можно определить операцию «возведения в степень» следующим образом. Определим, что , а для i > 0 определим как . Поскольку εs — просто s, то . Соответственно, , и так далее.

printf("Hell" "o World!" "\n"); // компилятор соберёт в "Hello World!\n"

...
// parameter types: by value VTs
#define VTS_I2              "\x02"      // a 'short'
#define VTS_I4              "\x03"      // a 'long'
#define VTS_R4              "\x04"      // a 'float'
#define VTS_R8              "\x05"      // a 'double'
#define VTS_CY              "\x06"      // a 'CY' or 'CY*'
#define VTS_DATE            "\x07"      // a 'DATE'
#define VTS_WBSTR           "\x08"      // an 'LPCOLESTR'
#define VTS_DISPATCH        "\x09"      // an 'IDispatch*'
#define VTS_SCODE           "\x0A"      // an 'SCODE'
#define VTS_BOOL            "\x0B"      // a 'BOOL'
#define VTS_VARIANT         "\x0C"      // a 'const VARIANT&' or 'VARIANT*'
#define VTS_UNKNOWN         "\x0D"      // an 'IUnknown*'
...

...
BEGIN_EVENTSINK_MAP(CHtmlView, CFormView)
  ...
  ON_EVENT(CHtmlView, AFX_IDW_PANE_FIRST, 105 /* CommandStateChange */, OnCommandStateChange, VTS_I4 VTS_BOOL)
  ...
END_EVENTSINK_MAP()
...
void CHtmlView::OnCommandStateChange(long lCommand, BOOL bEnable)
{
  ...
}
...