fix some functions for valid processing of empty string content (cstr_==0, len_==0)

modules/core/include/opencv2/core/cvstd.hpp

@@ -896,6 +896,7 @@ size_t String::find_first_of(const String& str, size_t pos) const
 inline
 size_t String::find_first_of(const char* s, size_t pos) const
 {
+    if (len_ == 0) return npos;
     if (pos >= len_ || !s[0]) return npos;
     const char* lmax = cstr_ + len_;
     for (const char* i = cstr_ + pos; i < lmax; ++i)
@@ -910,6 +911,7 @@ size_t String::find_first_of(const char* s, size_t pos) const
 inline
 size_t String::find_last_of(const char* s, size_t pos, size_t n) const
 {
+    if (len_ == 0) return npos;
     if (pos >= len_) pos = len_ - 1;
     for (const char* i = cstr_ + pos; i >= cstr_; --i)
     {
@@ -935,6 +937,7 @@ size_t String::find_last_of(const String& str, size_t pos) const
 inline
 size_t String::find_last_of(const char* s, size_t pos) const
 {
+    if (len_ == 0) return npos;
     if (pos >= len_) pos = len_ - 1;
     for (const char* i = cstr_ + pos; i >= cstr_; --i)
     {

modules/core/test/test_misc.cpp

@@ -129,3 +129,12 @@ TEST(Core_OutputArrayAssign, _Matxf_UMatd)
 
     EXPECT_LE(maxAbsDiff(expected, actual), FLT_EPSILON);
 }
+
+
+TEST(Core_String, find_last_of__with__empty_string)
+{
+    cv::String s;
+    size_t p = s.find_last_of("q", 0);
+    // npos is not exported: EXPECT_EQ(cv::String::npos, p);
+    EXPECT_EQ(std::string::npos, p);
+}