absl::StrFormat — type-safe printf·FormatSpec

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// 회피 — UB
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char buf[64];
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snprintf(buf, sizeof(buf), "id=%d", "string_not_int");  // type 불일치
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snprintf(buf, sizeof(buf), "%s", nullptr);              // glibc 외에는 UB
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snprintf(buf, 4, "%s", "too long");                     // 잘림 — 실수 잡기 어려움

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#include "absl/strings/str_format.h"
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namespace absl {
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template <typename... Args>
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std::string StrFormat(const FormatSpec<Args...>& format, const Args&... args);
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template <typename... Args>
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bool StrAppendFormat(std::string* dst, const FormatSpec<Args...>& format,
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                     const Args&... args);
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template <typename... Args>
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ABSL_MUST_USE_RESULT int SNPrintF(char* out, size_t n,
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                                  const FormatSpec<Args...>& format,
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                                  const Args&... args);
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}

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std::string s = absl::StrFormat("user=%d action=%s", 42, "login");
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// "user=42 action=login"
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std::string ip = absl::StrFormat("%d.%d.%d.%d", 192, 168, 0, 1);
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double pi = 3.141592653589;
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std::string f = absl::StrFormat("%.4f", pi);  // "3.1416"

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std::string name = "Alice";
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absl::string_view sv = "Bob";
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absl::StrFormat("hello %s and %s", name, sv);

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// 회피 — 컴파일 에러
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absl::StrFormat("id=%d", "not_int");
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// error: format string requires int, got const char[*]
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absl::StrFormat("%s", 42);
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// error: format string requires string-like, got int

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absl::UntypedFormatSpec spec("%d %s");
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std::string out;
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if (!absl::FormatUntyped(&out, spec, {absl::FormatArg(42), absl::FormatArg("hi")})) {
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  // 검증 실패 (인자 타입 불일치 등)
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}

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absl::ParsedFormat<'d','s'> spec("%d %s");
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for (auto [n, s] : items) {
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  std::string line = absl::StrFormat(spec, n, s);
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}

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// absl/strings/internal/str_format/extension.h (요약)
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class FormatRawSink {
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 public:
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  virtual void Write(string_view s) = 0;
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};
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class FormatSinkImpl {
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  std::string* buffer_;
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 public:
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  void Append(string_view s) { buffer_->append(s.data(), s.size()); }
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};

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// 회피
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char buf[256];
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snprintf(buf, sizeof(buf), "user=%d ip=%s ms=%lld", uid, ip.c_str(), ms);
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std::string s(buf);
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// Good
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std::string s = absl::StrFormat("user=%d ip=%s ms=%lld", uid, ip, ms);
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// .c_str() 불필요, 잘림 없음, type 검증

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LOG(WARNING) << absl::StrFormat("slow query: %.2f ms (threshold=%.2f)",
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                                 elapsed_ms, threshold_ms);

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int prec = 6;
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absl::StrFormat("%.*f", prec, 3.141592);  // "3.141592"

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// 회피 — UB / 보안 문제
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absl::StrFormat(user_supplied, ...);
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// Good
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absl::StrFormat("%s", user_supplied);

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// hot path
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absl::StrFormat("k=%d", n);   // 더 무거움
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absl::StrCat("k=", n);        // 더 가벼움