diff --git a/src/str.cpp b/src/str.cpp
index bcf5696..44cef1b 100644
--- a/src/str.cpp
+++ b/src/str.cpp
@@ -12,6 +12,7 @@ String8 push_str8fv(Arena *arena, const char *fmt, va_list args)
   return result;
 }
 
+internal
 String8 push_str8f(Arena *arena, const char *fmt, ...)
 {
   va_list args;
@@ -21,6 +22,7 @@ String8 push_str8f(Arena *arena, const char *fmt, ...)
   return result;
 }
 
+internal
 String8 str8_from_c(const char *str)
 {
   u64 size = strlen(str);
diff --git a/src/str.h b/src/str.h
index 0e5ceed..c521518 100644
--- a/src/str.h
+++ b/src/str.h
@@ -3,15 +3,14 @@ struct String8 {
   u64 size;
 
   const char *c() const { return reinterpret_cast<const char *>(str); }
-  u8 operator[](u64 idx) const { return str[idx]; }
+  u8 operator[](u64 idx) const {
+    assert(idx < size);
+    return str[idx];
+  }
 };
 
 #define str8(s) String8 { (u8*)(s), sizeof(s) - 1 }
 
-String8 str8_from_c(const char *str);
-String8 push_str8f(Arena *arena, char *fmt, ...);
-
-
 struct String8_Node {
   String8_Node *next;
   String8 str;