4da4f33 Aug 16, 2016
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folly/Conv.h is a one-stop-shop for converting values across types. Its main features are simplicity of the API (only the names to and toAppend must be memorized), speed (folly is significantly faster, sometimes by an order of magnitude, than comparable APIs), and correctness.


All examples below are assume to have included folly/Conv.h and issued using namespace folly; You will need:

    // To format as text and append to a string, use toAppend.
    fbstring str;
    toAppend(2.5, &str);
    CHECK_EQ(str, "2.5");

    // Multiple arguments are okay, too. Just put the pointer to string at the end.
    toAppend(" is ", 2, " point ", 5, &str);
    CHECK_EQ(str, "2.5 is 2 point 5");

    // You don't need to use fbstring (although it's much faster for conversions and in general).
    std::string stdStr;
    toAppend("Pi is about ", 22.0 / 7, &stdStr);
    // In general, just use to<TargetType>(sourceValue). It returns its result by value.
    stdStr = to<std::string>("Variadic ", "arguments also accepted.");

    // to<fbstring> is 2.5x faster than to<std::string> for typical workloads.
    str = to<fbstring>("Variadic ", "arguments also accepted.");

Integral-to-integral conversion

Using to<Target>(value) to convert one integral type to another will behave as follows:

  • If the target type can accommodate all possible values of the source value, the value is implicitly converted. No further action is taken. Example:
        short x;
        unsigned short y;
        auto a = to<int>(x); // zero overhead conversion
        auto b = to<int>(y); // zero overhead conversion
  • Otherwise, to inserts bounds checks and throws std::range_error if the target type cannot accommodate the source value. Example:
    short x;
    unsigned short y;
    long z;
    x = 123;
    auto a = to<unsigned short>(x); // fine
    x = -1;
    a = to<unsigned short>(x); // THROWS
    z = 2000000000;
    auto b = to<int>(z); // fine
    z += 1000000000;
    b = to<int>(z); // THROWS
    auto b = to<unsigned int>(z); // fine

Anything-to-string conversion

As mentioned, there are two primitives for converting anything to string: to and toAppend. They support the same set of source types, literally by definition (to is implemented in terms of toAppend for all types). The call toAppend(value, &str) formats and appends value to str whereas to<StringType>(value) formats value as a StringType and returns the result by value. Currently, the supported StringTypes are std::string and fbstring

Both toAppend and to with a string type as a target support variadic arguments. Each argument is converted in turn. For toAppend the last argument in a variadic list must be the address of a supported string type (no need to specify the string type as a template argument).

Integral-to-string conversion

Nothing special here - integrals are converted to strings in decimal format, with a '-' prefix for negative values. Example:

    auto a = to<fbstring>(123);
    assert(a == "123");
    a = to<fbstring>(-456);
    assert(a == "-456");

The conversion implementation is aggressively optimized. It converts two digits at a time assisted by fixed-size tables. Converting a long to an fbstring is 3.6x faster than using boost::lexical_cast and 2.5x faster than using sprintf even though the latter is used in conjunction with a stack-allocated constant-size buffer.

Note that converting integral types to fbstring has a particular advantage compared to converting to std::string No integral type (<= 64 bits) has more than 20 decimal digits including sign. Since fbstring employs the small string optimization for up to 23 characters, converting an integral to fbstring is guaranteed to not allocate memory, resulting in significant speed and memory locality gains. Benchmarks reveal a 2x gain on a typical workload.

char to string conversion

Although char is technically an integral type, most of the time you want the string representation of 'a' to be "a", not 96 That's why folly/Conv.h handles char as a special case that does the expected thing. Note that signed char and unsigned char are still considered integral types.

Floating point to string conversion

folly/Conv.h uses V8's double conversion routines. They are accurate and fast; on typical workloads, to<fbstring>(doubleValue) is 1.9x faster than sprintf and 5.5x faster than boost::lexical_cast (It is also 1.3x faster than to<std::string>(doubleValue)

const char* to string conversion

For completeness, folly/Conv.h supports const char* including i.e. string literals. The "conversion" consists, of course, of the string itself. Example:

    auto s = to<fbstring>("Hello, world");
    assert(s == "Hello, world");

Anything from string conversion (i.e. parsing)

folly/Conv.h includes three kinds of parsing routines:

  • to<Type>(const char* begin, const char* end) rigidly converts the range [begin, end) to Type These routines have drastic restrictions (e.g. allow no leading or trailing whitespace) and are intended as an efficient back-end for more tolerant routines.
  • to<Type>(stringy) converts stringy to Type Value stringy may be of type const char*, StringPiece, std::string, or fbstring (Technically, the requirement is that stringy implicitly converts to a StringPiece
  • to<Type>(&stringPiece) parses with progress information: given stringPiece of type StringPiece it parses as much as possible from it as type Type and alters stringPiece to remove the munched characters. This is easiest clarified by an example:
    fbstring s = " 1234 angels on a pin";
    StringPiece pc(s);
    auto x = to<int>(&pc);
    assert(x == 1234);
    assert(pc == " angels on a pin";

Note how the routine ate the leading space but not the trailing one.

Parsing integral types

Parsing integral types is unremarkable - decimal format is expected, optional '+' or '-' sign for signed types, but no optional '+' is allowed for unsigned types. The one remarkable element is speed - parsing typical long values is 6x faster than sscanf. folly/Conv.h uses aggressive loop unrolling and table-assisted SIMD-style code arrangement that avoids integral division (slow) and data dependencies across operations (ILP-unfriendly). Example:

    fbstring str = "  12345  ";
    assert(to<int>(str) == 12345);
    str = "  12345six seven eight";
    StringPiece pc(str);
    assert(to<int>(&pc) == 12345);
    assert(str == "six seven eight");

Parsing floating-point types

folly/Conv.h uses, again, V8's double-conversion routines as back-end. The speed is 3x faster than sscanf and 1.7x faster than in-home routines such as parse<double> But the more important detail is accuracy - even if you do code a routine that works faster than to<double> chances are it is incorrect and will fail in a variety of corner cases. Using to<double> is strongly recommended.

Note that if an unparsable string is passed to to<double> NaN is returned, which can be tested for as follows:

    fbstring str = "not a double";
    double d = to<double>(str);
    if (std::isnan(d)) {
      // string could not be parsed

Non-throwing interfaces

tryTo<T> is the non-throwing variant of to<T>. It returns an Expected<T, ConversionCode>. You can think of Expected as like an Optional<T>, but if the conversion failed, Expected stores an error code instead of a T.

tryTo<T> has similar performance as to<T> when the conversion is successful. On the error path, you can expect tryTo<T> to be roughly three orders of magnitude faster than the throwing to<T> and to completely avoid any lock contention arising from stack unwinding.

Here is how to use non-throwing conversions:

    auto t1 = tryTo<int>(str);
    if (t1.hasValue()) {

Expected has a composability feature to make the above pattern simpler.

    tryTo<int>(str).then([](int i) { use(i); });