>>33のcapture_stringをパクればいいんでね？

class CaptureString {
public:
　typedef char value_type;
private:
　CString *target;
　size_t size;
　value_type *buffer;
public:
　CaptureString(CString *target_, size_t size_ = DEFAULT_SIZE)
　　: target(target_), size(size_), buffer(NULL) {
　}
　~CaptureString() {
　　if (buffer != NULL) {
　　　target->ReleaseBuffer();
　　}
　}
　operator value_type *() {
　　if (buffer == NULL) {
　　　buffer = target->GetBuffer(size);
　　}
　　return buffer;
　}
};

// 使用例
CString str;
fgets(stdin, CaptureString(&str, BUF_SIZE), BUF_SIZE);