diff --git a/src/shrpx_config.cc b/src/shrpx_config.cc
index 43433f6a..a661fc03 100644
--- a/src/shrpx_config.cc
+++ b/src/shrpx_config.cc
@@ -542,7 +542,8 @@ std::vector<LogFragment> parse_log_format(const char *optarg) {
     }
 
     if (literal_start < var_start) {
-      res.emplace_back(SHRPX_LOGF_LITERAL, strcopy(literal_start, var_start));
+      res.emplace_back(SHRPX_LOGF_LITERAL,
+                       ImmutableString(literal_start, var_start));
     }
 
     literal_start = p;
@@ -552,17 +553,18 @@ std::vector<LogFragment> parse_log_format(const char *optarg) {
       continue;
     }
 
-    res.emplace_back(type, strcopy(value, var_name + var_namelen));
-    auto &v = res.back().value;
-    for (size_t i = 0; v[i]; ++i) {
-      if (v[i] == '_') {
-        v[i] = '-';
+    auto name = std::string(value, var_name + var_namelen);
+    for (auto &c : name) {
+      if (c == '_') {
+        c = '-';
       }
     }
+
+    res.emplace_back(type, ImmutableString(name));
   }
 
   if (literal_start != eop) {
-    res.emplace_back(SHRPX_LOGF_LITERAL, strcopy(literal_start, eop));
+    res.emplace_back(SHRPX_LOGF_LITERAL, ImmutableString(literal_start, eop));
   }
 
   return res;
diff --git a/src/shrpx_downstream.cc b/src/shrpx_downstream.cc
index c9f148b9..6fe16f9e 100644
--- a/src/shrpx_downstream.cc
+++ b/src/shrpx_downstream.cc
@@ -224,8 +224,8 @@ void Downstream::force_resume_read() {
 }
 
 namespace {
-const Headers::value_type *get_header_linear(const Headers &headers,
-                                             const std::string &name) {
+const Headers::value_type *search_header_linear(const Headers &headers,
+                                                const StringRef &name) {
   const Headers::value_type *res = nullptr;
   for (auto &kv : headers) {
     if (kv.name == name) {
@@ -395,8 +395,8 @@ Headers::value_type *FieldStore::header(int16_t token) {
   return http2::get_header(hdidx_, token, headers_);
 }
 
-const Headers::value_type *FieldStore::header(const std::string &name) const {
-  return get_header_linear(headers_, name);
+const Headers::value_type *FieldStore::header(const StringRef &name) const {
+  return search_header_linear(headers_, name);
 }
 
 void FieldStore::add_header(std::string name, std::string value) {
diff --git a/src/shrpx_downstream.h b/src/shrpx_downstream.h
index 7bc7f0ce..7e8c1f45 100644
--- a/src/shrpx_downstream.h
+++ b/src/shrpx_downstream.h
@@ -76,7 +76,7 @@ public:
   Headers::value_type *header(int16_t token);
   // Returns pointer to the header field with the name |name|.  If no
   // such header is found, returns nullptr.
-  const Headers::value_type *header(const std::string &name) const;
+  const Headers::value_type *header(const StringRef &name) const;
 
   void add_header(std::string name, std::string value);
   void add_header(std::string name, std::string value, int16_t token);
diff --git a/src/shrpx_log.cc b/src/shrpx_log.cc
index 84498b72..aacde3f7 100644
--- a/src/shrpx_log.cc
+++ b/src/shrpx_log.cc
@@ -243,7 +243,7 @@ void upstream_accesslog(const std::vector<LogFragment> &lfv,
   for (auto &lf : lfv) {
     switch (lf.type) {
     case SHRPX_LOGF_LITERAL:
-      std::tie(p, avail) = copy(lf.value.get(), avail, p);
+      std::tie(p, avail) = copy(lf.value, avail, p);
       break;
     case SHRPX_LOGF_REMOTE_ADDR:
       std::tie(p, avail) = copy(lgsp.remote_addr, avail, p);
@@ -273,7 +273,7 @@ void upstream_accesslog(const std::vector<LogFragment> &lfv,
       break;
     case SHRPX_LOGF_HTTP:
       if (req) {
-        auto hd = req->fs.header(lf.value.get());
+        auto hd = req->fs.header(lf.value);
         if (hd) {
           std::tie(p, avail) = copy((*hd).value, avail, p);
           break;
diff --git a/src/shrpx_log.h b/src/shrpx_log.h
index 84d509cf..7cb00fde 100644
--- a/src/shrpx_log.h
+++ b/src/shrpx_log.h
@@ -134,10 +134,10 @@ enum LogFragmentType {
 };
 
 struct LogFragment {
-  LogFragment(LogFragmentType type, std::unique_ptr<char[]> value = nullptr)
+  LogFragment(LogFragmentType type, ImmutableString value = ImmutableString())
       : type(type), value(std::move(value)) {}
   LogFragmentType type;
-  std::unique_ptr<char[]> value;
+  ImmutableString value;
 };
 
 struct LogSpec {
diff --git a/src/template.h b/src/template.h
index 3b617e54..259a19e1 100644
--- a/src/template.h
+++ b/src/template.h
@@ -218,6 +218,16 @@ inline std::unique_ptr<char[]> strcopy(const std::unique_ptr<char[]> &val,
 // appear before the final terminal NULL.
 class ImmutableString {
 public:
+  using traits_type = std::char_traits<char>;
+  using value_type = traits_type::char_type;
+  using allocator_type = std::allocator<char>;
+  using size_type = std::allocator_traits<allocator_type>::size_type;
+  using difference_type =
+      std::allocator_traits<allocator_type>::difference_type;
+  using const_reference = const value_type &;
+  using const_pointer = const value_type *;
+  using const_iterator = const_pointer;
+
   ImmutableString() : len(0) {}
   ImmutableString(const char *s, size_t slen)
       : len(slen), base(strcopy(s, len)) {}
@@ -244,10 +254,10 @@ public:
   }
 
   const char *c_str() const { return base.get(); }
-  size_t size() const { return len; }
+  size_type size() const { return len; }
 
 private:
-  size_t len;
+  size_type len;
   std::unique_ptr<char[]> base;
 };
 
@@ -258,10 +268,19 @@ private:
 // function can be used to export the content as std::string.
 class StringRef {
 public:
+  using traits_type = std::char_traits<char>;
+  using value_type = traits_type::char_type;
+  using allocator_type = std::allocator<char>;
+  using size_type = std::allocator_traits<allocator_type>::size_type;
+  using difference_type =
+      std::allocator_traits<allocator_type>::difference_type;
+  using const_reference = const value_type &;
+  using const_pointer = const value_type *;
+  using const_iterator = const_pointer;
+
   StringRef() : base(""), len(0) {}
-  template <typename T>
-  StringRef(const T &s)
-      : base(s.c_str()), len(s.size()) {}
+  StringRef(const std::string &s) : base(s.c_str()), len(s.size()) {}
+  StringRef(const ImmutableString &s) : base(s.c_str()), len(s.size()) {}
   StringRef(const char *s) : base(s), len(strlen(s)) {}
   StringRef(const char *s, size_t n) : base(s), len(n) {}
 
@@ -269,16 +288,31 @@ public:
     return StringRef(s, N - 1);
   }
 
+  const_iterator begin() const { return base; };
+  const_iterator cbegin() const { return base; };
+
+  const_iterator end() const { return base + len; };
+  const_iterator cend() const { return base + len; };
+
   const char *c_str() const { return base; }
-  size_t size() const { return len; }
+  size_type size() const { return len; }
 
   std::string str() const { return std::string(base, len); }
 
 private:
   const char *base;
-  size_t len;
+  size_type len;
 };
 
+inline bool operator==(const StringRef &lhs, const std::string &rhs) {
+  return lhs.size() == rhs.size() &&
+         std::equal(std::begin(lhs), std::end(lhs), std::begin(rhs));
+}
+
+inline bool operator==(const std::string &lhs, const StringRef &rhs) {
+  return rhs == lhs;
+}
+
 inline int run_app(std::function<int(int, char **)> app, int argc,
                    char **argv) {
   try {